Hi Abhishek,

1.A large number of boys were absent on account of bad weather.

2.The number of admissions have gradually fallen off.

Was/were is the plural past tense of the verb to be. We know to use plural becasue boys has an s on the end meaning there is more than one boy so we use the plural conjugation of the verb to be which is were.

present tense = I am, you are, she is, they are, we are
past tense = I was, you were, she was, they were, we were

they is the pronoun we use when conjugating the plural, (they means more than one) and we are speaking in the past tense, so the conjugation of the verb "to be" is were.

admissions is plural, we know this because admission has an s on the end, the s lets us know we are talking about more than one admission.

the verb to have is conjugated like this

present tense = i have, you have, she has, they have, we have 

there is more than one admission so we use the pronoun they(meaning more than one) to help us conjugate.

I has = singluar, they have = plural, admissions have = plural

I hope that helps.