Question

when he entered the classroom the lecture already was begining​

Answer 1

what we have to do please tell me or write it again

Answer 2

When he entered the classroom the lecture had already begun.

Simple Past and Past Perfect are used together in a sentence when we are speaking of two actions in the past that took place at different times. We use PAST PERFECT tense for the previously concluded action i.e. for the action that happened earlier. Past Perfect tense is HAD +V3 form. For the other activities in the past, we use SIMPLE PAST i.e. V2 form

Here, the second action took place before the first one. So we use past perfect tense for the second action ( the lecture had already begun ) and use simple past for the first one ( When he entered the classroom ).