So far I know, we also don't use auxiliar verb when there is no subject, for example. What happened? Why to run if there is no one running after us? I'm not specialist in English, so if I'm wrong, please correct me.

"To ask questions with..., DO YOU always add "do" or "does"?" You can only add "do" or does" to a sentence if it doesn't have an auxiliary verb. If a sentence doesn't have an auxiliary verb, you can choose to add "do" or "does" for emphasis. E.g. "He plays outside." = "He does play outside." "He rides a bike." = "He does ride a bike." The only difference is that the latter is only used for emphasis. You only NEED to add "do" if: 1) the sentence is a question 2) the sentence is a negative sentence You can't say "He do can..." or "They do have gone...", because "can" and "have" are auxiliary verbs. Questions in English are made by changing the word order from SVO to VSO. E.g. "I can do that." => "Can I do that?" "He has been there." => "Has he been there?" "You could find it." => "Could you find it?" "She is very tall." => "Is she very tall?" The catch is that if a sentence has an emphasised form, you HAVE TO use that. E.g. "You always add it" is a statement. You can't say "Add you always it?" because "add" isn't an auxiliary verb. You instead have to apply the normal rules of making questions to the emphasised form ("You do always add it."). This gives you "Do you always add it?". Whenever you want to make a question in English, you need to follow the following steps: 1) Does the sentence already have an auxiliary verb? If yes, skip to step 2. If no, skip to step 3. 2) Change the word order. Add a question to the beginning of the sentence if you want one. You're done! 3) Add "do" to the sentence to make it an auxiliary verb. Conjugate "do" appropriately. 4) Go to step 2.

No, not necessarily. Modals and linking verbs work too. E.g. "Where is it?", "Who gets the prize?", "When can you come?", "What became of him?", ...

No. We only use 'do' or 'does' as an auxiliary verb for questions in the PRESENT SIMPLE. We use 'did' for questions in the PAST SIMPLE. It is not necessary to use do/does/did for other tenses, because all other tenses already have auxiliary verbs (like 'have' or 'is') or a modal verb (like 'will' or 'can'). For all other tenses, all you need to do to form a question is reverse the order of the subject and the auxiliary verb. Here are some examples: He is eating -----> What is he eating? He was eating -----> What was he eating? He has eaten ------> What has he eaten? He will eat ------> What will he eat? As you can see, the word order is always the same, QASV: 1. Q: Question word - What 2, A: Auxiliary verb - is 3. S: Subject - he 4. V: Main verb - eating? Simple tenses are formed with a single verb form - 'He eats' or 'He ate'. There is no auxiliary verb to go in position number 2 of the QASV. It is not grammatically possible to say 'What he eat?', so we add 'do' to fill in this gap. He eats ---> What does he eat? He ate ---> What did he eat? I hope this helps.

Part 2: E.g. "He is tired" - This already has an auxiliary verb, so you only need to change the word order. =>"Is he tired?" "He can see something." - "Can" is an auxiliary verb. =>"Can he see something?" If you want, you can then say something like "Why can he see something?" or "How can he see something?". "He plays tennis." - "To play" is not an auxiliary verb, so "Plays he tennis?" wouldn't be correct. =>"He does play tennis." - Now that the sentence has an auxiliary verb, you can change the word order. =>"Does he play tennis?" If you want then you can add "How" or "Why" to the beginning. "Do" and "have" can be tricky because they're only sometimes auxiliary verbs. "Do" is only an auxiliary verb before another verb, and "have" is only an auxiliary verb when it's part of a perfect tense. E.g. "He has a cat." - "Has" is not an auxiliary verb here, so "Has he a cat?" isn't correct in (modern) English. =>"He does have a cat." - Now the sentence has an auxiliary verb. =>"Does he have a cat?" "He has gone to work" - "Has" is an auxiliary verb here, so "He does have gone..." (and thus, "Does he have gone...?") isn't correct. You can simply change the word order. =>"Has he gone to work?"