past continuous

[English4everyone]

I was thinking about what I had to buy from that market.
I was thinking about what I should buy from that market.
Which one is correct? Maybe both of them and it depend on the situation?

[5jj]

I was thinking about what I had to buy from that market.
I was thinking about what I should buy from that market.

Both are grammatically OK.

[English4everyone]

How?
Sorry, I can't undersatnd easily. May I ask you to explain it?
Thanks very much.

[emsr2d2]

How?
Sorry, I can't undersatnd easily. May I ask you to explain it?
Thanks very much.

Do you think that they are not grammatically OK?

[English4everyone]

No I think because we are talking about past and here ''thinking'' happens in the past, we have to use had to. How is it correct to use should? Just in some special situations?

[emsr2d2]

"Should" is still "should" even when referring to the past.

I wonder what I should do.
I wondered what I should do.

It is not the same for "have to", meaning "must".

I wonder what I have to do.
I wondered what I had to do.

[English4everyone]

Thanks a lot.
But my teacher said the past form for have to, has to, should & must is had to. So it isn't correct?

[emsr2d2]

Thanks a lot.
But my teacher said the past form for have to, has to, should & must is had to. So it isn't correct?

It entirely depends on the context. In your sentence, the verb which is moved into the past tense is "to wonder", followed by "should". If the sentence had been:

I should buy some apples. (Present tense)

...it would actually be quite difficult to change that into the past tense. In my opinion, again, saying "I had to buy some apples" is not a past tense version of "I should".