It is not our aim here to recapitulate the rules of the grammar book but we want to emphasise that your thesis must demonstrate an extremely advanced level of written English. This means in practice that you must ensure that spelling, grammar, vocabulary, punctuation, syntax, paragraphing and register are all correct. If you are writing your PhD in one of the Romance languages because the subject of your thesis is in that language (which you may be entitled to do: check your university regulations) then the level of written fluency, be it in French, Italian, Spanish or Portuguese, must also observe the same high standards. Examiners frequently complain about these sorts of slips: nothing irritates them more than bad writing. So if you know that you have problems with any of these elements of language, then take steps to improve them. Your supervisor will help you identify these sorts of problems and be able to suggest ways to tackle them. Do not rely on the services of a proof-reader, friend or professional, to eliminate them at the very end – it is almost impossible to correct the language of an entire thesis and keep it intact. Nor should you entirely rely on your computer spell-check tool. Favourite spell-check oversights are for example there and their. Treblecheck any use of foreign language, e.g. quotes and titles, but also proper names. You can see why examiners get aggravated by mistakes: if you are writing about writing, and your own language is faulty, it undermines the reader’s trust in what you have to say, and also distracts them from your argument, if indeed they can penetrate to it through the forest of little mistakes.
So: if English is not your mother tongue, try to get your supervisor’s and native speakers’ opinions of your writing – their reading of register and style may be more finely tuned than yours. If they don’t think it “sounds quite right,” then try to do something about it. Don’t rely on them to correct it (they may not understand or be able to communicate what is wrong): get some specialist help. Probably your supervisor will not be the best person here: teaching you English is neither their job nor probably their area of expertise. Your university or college will almost certainly run courses in English as a second language, on academic writing for non-native speakers, etc. Turn to the specialists to help you improve.
People often don’t study the elements of language and style in an undergraduate degree. They may often have had feedback from tutors about style and syntax without being sure what the problem was or how to improve it. When the time comes to do an extended piece of writing, those problems may emerge more clearly. Try to get frank opinions from your supervisor and others about the correctness of your English and about the readability and register of your style. A relatively minor problem, such as the rules governing the use of the colon and the semi-colon, is easy to correct by consultation of the appropriate reference work. Merely by identifying a problem, you are often half way to solving it. If you have more deeply-embedded difficulties with the mechanics of writing, then you may find it helpful to attend some sort of writing course in academic or perhaps even creative writing. Ultimately, how you tackle a problem such as this is entirely up to you, but do be aware that accuracy, fluency and style all contribute in important ways to the impression your thesis gives, and thus to the way in which it is assessed. Writing skills are also something that you can take away with you from an MA or PhD: they are highly valuable and highly valued even outside academia, so don’t miss this opportunity to develop them.