Complex verb tenses: 3 or more verbs together,modals.

Compound verbs are formed by putting two or three verbs together, one or two helping, or auxiliary verbs with a participle, or main verb. They are similar to compound nouns, which put two nouns together, such as "raincoat," except that with compound verbs, the verbs are written as separate words. Compound verbs are commonly used in English, in such constructions as future, continuous and perfect tenses, the passive voice and with modal verbs.

 Modal verbs can only be used in compound verbs.Examples of modals are can, could, will, should, could and would. They can be used to make requests, or talk about possibility, as with "can"; to talk about possibility or add politeness, as with "could"; or give advice, as is the case with "should." Not all modal verbs can be used in all tenses. They are used together with participles, as in "Jason could have gone,"

 

 
The English lexicon contains a few true compound verbs, such as stirfry, kickstart and forcefeed. These are not serial verbs, though, as with many compounds, they may be spelled as two words. Rather the first verb expresses a manner with which the action expressed by the second verb is carried out. The second verb is the only one which may express tense.  English also expresses fine distinctions as to the beginning, duration, completion, or repetition, of an action using auxiliaries or other lexical mechanisms. Examples here include was starting, had lived, had been seen, etc.