1.

a) She has been teaching these children for 5 months

- She taught these children 5 months ago

b) He wrote me a letter in April

-> He hasn't written me a letter since April

c) When was the last time you talked to him?

->How long has it been since you last talked to him?

d) It's been a long time since we last met

-> We last met a long time ago

2. 

a) I find this picture beautiful

b) I find it difficult to understand my father

c) I find that Maria is a hard-working student

3. 

a) I used my computer 2 years ago

b) They bought the shoes last month

a. Mary asked what I would do

b. The teacher said that he would play football with me. 

1. 

a) She had taught these children for 5 months.

She taught these children 5 months ago and now she still teaches them.

b) He has written me a letter since April.

He wrote me a letter in April.

c) How long have you talked to him for the last time?

When was the last time you talked to him?

d) It has been a long time since we last met.

It was a long time that we did not meet.

2. 

a) I find this picture beautiful.

b) I find it hard to understand my father.

c) I find Maria hard-working