Ago

Typical mistakes: 

I met her ago three years. 

I have left school three years ago. 

I have started working for this firm before three years. 

My father died for three years. 

1. 'Ago' is placed after an expression of time, nor before it. 

I met her three years ago. 

a long time ago. 

2. 'Ago' is used with a past tense, not the present perfect. 

I left school three years ago. 

3. Do not confuse the adverbs 'ago' and 'before', 'Ago' is used when the 'point of reference' is the present: it means 'before now'. 'Before' is used when the point of reference is not the present: it means 'before then', 'earlier'. Compare the following: 

I started working for this firm three years ago. 

Last summer, I finally left the firm that I had joined eighteen years ago before. 

4. 'Ago' and 'for' are not the same. 'Ago' tells you how long before the present something happened; 'for' tells you how long an action or situation lasted. Compare the following: 

My father died three years ago (= three years before now). 

He was ill for three years before he died (his illness lasted three years).