Writing a term paper or a court filing? Skip contractions; your reader expects expanded forms in those strict formal registers. A client report is a different case: whether contractions fit depends on your organization's conventions, so check your house style before relaxing the rule. Some business writing guides now allow them, while others still call for expanded forms, which is why the house style check comes first.
Writing for international readers? Expand it's and similar forms, which can be misread as possessives or as is, in technical or business documents.
When your reader expects no contractions
You're writing an academic essay. Contractions usually read as too casual here, so most guides suggest the expanded form as your default. The advice isn't absolute: the MLA Style Center notes that contractions may not be suitable for all types of formal writing, which leaves room for judgment. In practice, expanding them is the safe choice. Instead of the results don't support the hypothesis, write the results do not support the hypothesis.
You're drafting a lab report or technical document. These registers expect expanded forms. Instead of it isn't possible to calibrate the instrument, write it is not possible to calibrate the instrument.
You're preparing a court filing or legal document. Keep contractions out. Instead of the defendant didn't appear, write the defendant did not appear.
You're writing a report to a client under strict style guidance. Cambridge Dictionary's guidance treats contractions as usually not appropriate in formal writing, so we can't confirm the figures becomes we cannot confirm the figures.
You're quoting someone directly in academic or journalistic prose. The quoted speech keeps its contractions. The witness said, I don't remember the date. The quoted words keep their contraction; the surrounding sentences stay expanded.
You're writing routine business email or a memo. Permissive guides now treat contractions as acceptable in business writing, so we're reviewing your order can work. The Gregg Reference Manual still treats contractions as informal, allowing only verb-phrase contractions in business communications. Check the applicable house style before you relax or tighten the rule.