Ago and for

Ago is used to say when something happened. For is used to say how long it lasted.

• He died three years ago. (= three years before now.)
• He was ill for five years before he died. (= His illness lasted five years.)
• I met him two days ago.
• I have known him for many years.

Ago and before

Ago is used to count back from the present. Before is used to count back from a past moment.

• I met him two weeks ago. (= two weeks before now)
• When we got talking, I realized that I had seen her ten years before.

All and both

All refers to three or more items.

• I will take all three books.
• Both refers to two items.
• I will take both books. (NOT I will take all two books.)

Alright and all right

All right is the standard spelling. Alright is common but it is often considered incorrect.

• She felt all right in spite of her accident. (Formal)
• She felt alright in spite of her accident. (Informal)

Avoid the spelling alright in term papers or job application letters.

Allow and let

Allow is followed by an object + infinitive with to.

• Please allow me to go.
• Please allow me to use your computer for a minute.
• Please allow me to sit here.

Let is followed by an object + infinitive without to.

• Please let me go.
• Please let me use your computer for a minute.
• Please let me sit here.