Main and Helping Verbs

Front

Who is knocking at my door? (present progressive)

Back

is knocking

Front

Jody's train arrived before noon. (past perfect)

Back

had arrived

Front

He memorizes the train schedule. (present perfect)

Back

had memorized

Front

He waited for over an hour. (future perfect)

Back

will have waited

Front

Main verb in 'Who is knocking at my door?'

Back

knocking

Front

Helping verb in 'Who is knocking at my door?'

Back

is

Front

Jody's train had arrived before noon. MAIN VERB?

Back

arrived