Identify the non-finite verb in the given sentence:
All the teachers have gone to the picnic together.
a) Have
b) Gone
c) To
d) together

Hint:
Remember that the verbs that do not explain the tense of the sentence and do not agree with the subject are called non-finite verbs.

Complete answer:
We know that verbs are action words and in English grammar, there are mainly two types of verbs in English – finite and non-finite. Finite verbs change their forms when there is a change in the number or person of the subject. Finite verbs also have different forms in different tenses. Eg. wants, couldn't solve, etc.
We should remember that non-finite verbs do not change their form when the number or person of the subject changes. There are mainly three types of non-finite verbs: infinitives, gerunds, and participles. Eg. to pass, to cross, etc.
Now we shall go through all the options one by one to deduce the correct answer.
We see that in option ‘a’, 'Have' explains the tense (present perfect tense) of the sentence. It is a finite verb. Hence, option ‘a’ is incorrect.
In option b, though 'gone' is the past participle form of the verb, it is a non-finite verb as it doesn't exhibit the tense of the subject/action of the sentence.
'To' is used as an adverb, infinitive marker, and a preposition. As it is not even a verb, 'to' is not a non-finite verb. Hence, option c is also incorrect.
Looking at the last option, 'Together' is an adverb and not a verb, so option d is also incorrect.
So, the correct answer is “Option b”.

Note:
Read the rules of a grammar given above carefully and then apply rules one by one to all the options given.