Martin's guide to paper grammar

This page summarizes some principles on grammar that I've passed on to students individually over the past few years.

It is not intended to be a substitute for careful reading or a usage guide: it just summarizes a few points that are relevant to writing academic papers in astronomy.


Verbs in English have two voices: the active and the passive.

Active examplePassive example
We analysed the dataThe data were analysed
Bloggs et al. (2008) showed...It was shown by Bloggs et al. (2008)..
In this paper we present...In this paper ... is presented

Using the passive voice de-emphasises the person or thing who did what you're describing. In some cases, as in example (1) above, it makes it unclear who actually carried out the action that's being described. It used to be the case that people were advised to write scientific papers largely in the passive voice. While I don't think you need to try to avoid it altogether (sometimes it's useful, as in the previous sentence) I feel strongly that you should write in the active voice when you are describing what you or we have done in a paper. I.e., we analysed the data, not the data were analysed; we show that... not it is shown that.... Using the passive here is at best cumbersome and at worst actually makes it unclear who did what.


If you are writing a multi-author paper, use 'we' and 'us'. If you are writing a single-author paper (or a thesis!), possible strategies include

I prefer the third option but opinion is split.

Note that you are allowed to use 'we' about papers which do not have the same set of authors, so long as there is some overlap. (We have shown (Smith & Jones 2005)... is OK as long as the current authors include either Smith or Jones.)

Referring to papers

Papers are people. That is, when you cite a paper, you are referring to people who have written a paper, not to a paper written by some people. Smith & Jones (2005)... is shorthand for Smith and Jones, in their paper published in 2005, .... You can tell that this is so because the verb that goes with the paper agrees with the number of authors: so we say 'Smith & Jones (2005) argue' (not 'argues'). Some examples of the difference this makes:

the data presented in Smith & Jones (2005)The data presented by Smith & Jones (2005)
It was shown in Bloggs et al. (2008) thatBloggs et al. (2008) showed that


This is a very confused area. To start with, I'll introduce the main tenses used in academic writing.

Present tense

This describes an action that is happening now or a fact which is independent of time.

In this paper we presentWe are presenting it right now
Centaurus A is a strongly variable X-ray sourceFact

Simple past tense (preterite)

This describes an action that happened in the past, or something that was true but may now not be, or any assertion about the past that comes with a past date or time associated with it.

We processed the data in the standard manner.Took place in the past
Centaurus A varied strongly between 2003 and 2007.Statement about past facts, date in past

Perfect tense

This describes the state of things now as a result of an event that took place in the past. (An important consequence of this is that statements in the perfect tense can't have times in the past associated with them. I have been to Paris is a statement about my state now as a result of having been to Paris in the past. *I have been to Paris in 2008 is ungrammatical because it refers to a specific past event. Native speakers of languages that lack a true perfect tense need to learn this rule.)

In scientific writing, we use the perfect to make statements about past events that affect us at the time of writing.

We have previously shown that...... and so now we are able to do something with that result.
In their earlier work Bloggs et al. have arguedSets the scene for what we're saying now

The use of the perfect flags up the fact that these past events are relevant to the present discussion.

Future tense

Fairly obvious; formed with 'will' and 'shall'. (The rule that 'shall' is the non-emphatic first-person future is more or less dead, but you should feel free to use it if you like: 'in this paper we shall argue that...'.)

What tenses should you use?

Basically use any you like, as long as they are coherent (make sense) and consistent (follow some logical rule which you obey throughout the paper). Consistency is usually the problem: for example, a very common error is to oscillate between the present and preterite tenses in describing the work you've done in the paper. Here's my suggestion for a logical scheme for a standard data + interpretation paper, though.


Use the present tense when describing the work you are doing now. (In this paper we present... we show that... we argue. Use the preterite for facts about the past that you need to introduce (Cen A was strongly variable during our observations). Use the perfect for any retrospectives on your or other people's work that you need to give (note that it is not usually a good idea to cite papers, yours or others', in an abstract).


Use whatever combination of tenses is appropriate to describe earlier work, but use the present (in particular, not the future) tense, as for the abstract, in describing any new work you are doing in the paper being written.

Analysis and discussion

When describing your data analysis, processing, calculations, simulations etc. use the preterite. From the perspective of the writing of the paper, you are describing events that happened in the past. (We processed... we used the AIPS software package to image... we fitted a Gaussian... we carried out a K-S test....) Use the present to describe something you are doing as you write: We conclude that... We prefer an interpretation in which.... Remember that the paper is written from the idealized perspective in which you have done all the work before you start writing!

Summary and conclusions

When reviewing discussion that took place earlier in the paper, it may be appropriate to use the perfect or the preterite (We have argued that... In Section 5 we found...). However, a statement of the paper's conclusions should be in the present.


The most common error here is to use a comma to separate connected sentences:

We were unable to solve the problem by doing X, it was necessary to do Y.

This is called a comma splice and is wrong in formal English. Sentences must either be joined by a connecting word (and, but, so, whereas...) or by a semicolon ;, or they must be separated by a full stop. The semicolon should be used whenever you want to indicate that two sentences are connected without using some explicit word to connect them. That word may not be at the join between sentences! For example,

Since we were unable to solve the problem by doing X, it was necessary to do Y.
is fine, because 'since' indicates the connection between the two sentences.

Note a special rule for the connecting word 'however':

When we did X we obtained result Y, however when we did P we found QWrong
When we did X we obtained result Y; however, when we did P we found QRight

'However' as a connecting word always starts a new sentence; it must be preceded by a semicolon and followed by a comma.


Try to avoid leaving these at the end of a sentence if you can, in formal writing: prefer 'the data from which we extracted the spectrum' to 'the data we extracted the spectrum from'.


In formal English you should generally use 'that' wherever it can be used to introduce a dependent clause following a verb: so

We observed the target was variableWrong
We observed that the target was variableRight

that vs which

In British English, 'which' can be used to introduce either defining or non-defining relative clauses, but 'that' can only be used to introduce defining ones:

The object which we observed was variableDefining — which object was it?
The object that we observed was variableDefining
The object, which we observed several times, was variableNon-defining — adds information; note commas
The object, that we observed several times, was variableWrong!

A 'rule' has grown up (and is particularly observed in US English) under which one should use 'that' for defining clauses and 'which' for non-defining clauses; i.e., the first example above would be 'corrected' to the second. This is a rule of writing not of grammar, but it is prevalent enough that it is probably worth following since it will avoid editorial changes. However, an unfortunate consequence of this approach is that people conclude that 'which' is wrong altogether, and produce sentences like the fourth example, which is simply ungrammatical. Awareness of the difference between the two sorts of clauses is important here.

(Side note: in referring to people, 'which' should be replaced by 'who' or 'whom'. 'The man that I saw' or 'The man whom I saw' are both grammatical. 'I saw a man who was wearing a hat' is fine but 'I saw a man that was wearing a hat' is not.)