I had to be very quiet and help dad look after her be是什麼時候會用到?

Have to =must ，所以其後用原形動詞be，否則直接寫I am very quiet. 即可，因強調必需如此，所以此處用be動詞，一般我們稱be動詞為連繫動詞，將前後做一連接，用法如連接詞或介繫詞等，本身無意思，功能類似黏着劑，將一句字變得前後可連結

be=linking verb, between the subject and various complements;
eg:-She is a baby-sitter.