What is model words?

modal verbs:
- like auxiliary verbs
-functions
1 ability(can, could, be able to)
2 possibility(may, might, can, could)
3 permission(can, could, may, might)
4 request(can, could, will, would)
5 necessity(must, have to, have got to)
6 advice(should, had better, need, have to)
-no present and past participle
-have past tense only
-very important
-suitable tone depends on them mainly

Examples:
You need to stop now! (giving advice)
You must stop now! (having an order)
You can solve the problem. (about the ability)
You should solve the problem. (giving advice)
You must solve the problem. (having a request)

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Auxiliary modal verbs also are include as modal verbs. They are as following: can, could, will, would, may, might, shall, should, must. All the above modal verbs should follow without “to” + “infinitive” (verb in present tense)For example: He must wear uniform to school (NOT: He must to wear uniform to school) If we need to ask a question or write a negative sentence MUST NOT start with “do” or “does”. For example: 1. May I listen to the phone? (NOT: Do I may listen to the phone?)2. Would you mind to share your book with me (NOT: Do you would mind to share your book with me)3. Should you cross the road when the traffic light turns to green? (NOT: Do you should cross the road when the traffic light turns to green)4. Paul cannot run for 10km (NOT: Paul does not can run for 10km) Has, have and need are categorized as modal verbs. However, unlike the auxiliary modal verb, these two modal verb MUST FOLLOW with “TO” + “INFINITIVE” (Verb in present tense). 1. John has to pass all his examinations in order that his parent will give him presents.2. Wilson needs to write up his report before leaving the office.
For has, have and need, when asking a question or form a negative meaning sentence, they are acting like normal verb that uses “do” or “Does”. For instance: 1. Does he have to drink medicine today? (NOT: Has he to drink)2. Clinton doesn’t need to go to badminton courses tomorrow. (NOT: Clinton need not go to…)

Remark: the above explanation is originated from me, not copying from other websites or books.