As well as the other suggestions, you can also do it with subjunctives: I insist that he see a doctor. I asked that she do it a gain. This sounds very formal, however, and many natives would add an s or use a different structure. I insist that he sees a doctor. I asked her to do it again.

Great question! Normally, in the present simple tense, verbs take an -s when used with he, she, or it (e.g., She runs every morning). However, there are cases where you do not add an -s, apart from negations and questions. Here are the key situations: 1. Modal Verbs When using modal verbs (can, could, may, might, must, shall, should, will, would), the main verb never takes an -s, even with he, she, it. Example: • She can swim. (Not: She can swims.) • He must finish his work. (Not: He must finishes.) 2. Imperative Sentences (Commands) Commands use the base form of the verb, regardless of the subject. Example: • Someone help her! (Not: Someone helps her!) • Let him go. (Not: Let him goes.) 3. After “Do” in Emphatic Statements When emphasizing an action, do is used in its base form, and the main verb remains without -s. Example: • She does know the answer! (Not: She does knows!)

after modal or auxiliary verbs ( can, could, would, should, ought to, will, have to, etc )