I am writing my thesis (in Computer Science, if that's relevant) and I am thinking about the style, especially about using contractions.
I realize that thesis is a formal text and contractions like “we're” are quite informal, so they shouldn't be used. But does that apply to all contractions (like “can't” or “don't”)?
English is not my first language, so I'm not sure how much informal the various contractions are.
Many people have different opinions, even among those who are native English speakers and/or think a lot about what makes for good exposition. That said, here are a few rules of thumb:
Your thesis is possibly the most formal writing you will ever do. Survey articles and expository articles (especially for undergrads or other non-experts) are often written less formally. Even many conference proceedings and some journal articles omit some details, and thus can feel less formal than your thesis.
No one will fault you for avoiding contractions altogether. If in doubt, leave it out (the contraction, that is). I hate some techniques common in formal writing, such as overuse of the passive voice, or nearly any use of the pronoun one. But lack of contractions doesn't bother me.
As a more general resource for non-native English speakers, consider Doug West's The Grammar. West has written two textbooks and over two hundred papers, as well as having served as a problem editor for the Math Monthly for the last 20-something years. Surely many people will disagree with at least one piece of advice he gives, but what I find helpful is that West explains his motivation for each piece of advice he offers.
The best advice depends upon the style of your thesis. If you are working in a field where you might be expected to publish your thesis, then something aimed at experts should be written in a formal tone (avoiding contractions whenever possible, outside of direct quotations). However, if you're aiming at a wider audience, then a more conversational tone might be completely appropriate. The same logic applies if the chapters of your thesis are planned for publication.
However, you should also check with your advisor about his or her expectations when writing. It would be better to get a sense of what will be allowed before you get too far along; major structural changes are always a pain afterwards.
If you are Lieutenant Commander Data: your brother can but you can't (or, as you would say, "cannot"). Otherwise: I see that you can use contractions in your informal writing, so it would be very strange if you were not able to use them when writing your thesis. If you can't, maybe contact Oliver Sacks: he should get at least an article about this, and maybe part of a book.
More seriously: obviously you can. Should you? Speaking as someone who has both written and carefully read theses in a STEM field (mathematics): one generally needs to worry first that the content is complete and correct and second that the writing is good enough so as not to detract (or distract) from the content. The use of contractions would be at least a level below anything I would worry about when reading a thesis...provided they are used correctly.
It's best to stick to language within your comfort zone (especially as a non-native speaker). But your post above indicates to me that you have more than enough facility with English to pull off contractions if you want to. You'll be fine.
