请问since,for,by的现在完成时时间状语引导词的区别及用法,

since + 句子（谓语动词为终止性动词的过去式/延续性动词的过去式）
It has been 3 years since he left home
It has been 3 years since he was at hme 自从他离开家已有3年了.
since + 过去时间/时间段 ago
He has been here since 1990.
He has been here since 10 years ago
for + 时间段
He has been here for 10 years.
by +now/现在时间（this )
He has taught in our school for 3 years by this month / by this year.