Yes, you can start a sentence with a verb.
action verb
The subject is often near the start of a sentence: it is the thing or person that the sentence is about, and for an action verb, it is the thing or person performing the action. The verb is the action or state described in the sentence: what the subject does or is.
The verb in the sentence "How are they different" is "are."
Is is the verb in your sentence.
The verb in that sentence is "are".
Were is the verb in that sentence.
Depending on the context, sentence is already a verb For example, "to sentence someone" is an action and therefore a verb.
'Needs' would be the verb in this sentence.
Yes, it is possible to have a sentence with a verb and without a helping verb. For example, "He runs every day" is a sentence that contains the verb "runs" without a helping verb.
Ran is the verb in that sentence>
the verb in the sentence is has
There is no verb in that fragment - it is not a sentence.