一d好easy既grammer........

你說的是 infinitive ( 不定式 )
e.g. We began to walk down the road .
句中 main verb 是 began ( past tense )
to walk 是 infinitive ,
infinitive 一般是用原形動詞 , 即不用改變 tense ,
它的 tense 是跟隨 main verb 的 ,
即是說 main verb 是 past tense , 它自然亦是 past tense , 但你不用改動它 ,

infinitive 有進行式的 ,
e.g. He seems to be following us . = It seems that he is following us .
seems 是現在式 , 故 to be following 是現在進行式 ,
e.g. He seemed to be following us . = It seemed that he was following us .
seemed 是過去式 , 故 to be following 是過去進行式 ,

infinitive 亦有完成式的 ,
e.g. He pretended to have read the book . = He pretended that he had read the book .
pretended 是過去式 , 故 to have read 是過去完成式 ,

infinitive 還有完成進行式的 ,
e.g. He seems to have been spying for both sides .

除了infinitive ( with to ) 外 , 助動詞後面的 bare infinitive ( without to ) 亦是用原形動詞 ,
助動詞包括 :
may , might , can , could , shall , should , must , needn' t , will , would , 等等 ,

還有 feel , hear , see ,watch , let , make , would rather , sooner than 等等 ,
後面動詞亦是 bare infinitive ,

can都好似係= ="

你的問題不詳細, 很難回答你