Mixing tenses in subjunctive?

Question

I was presented with the following sentence and am wondering if this is formed correctly. I had learned that you should not mix tenses but in this case it is using past (albeit preterite and imperfect).

No creo que hice lo mejor que pudiera hacer.

I was reading about the past perfect subjunctive forms and am also wondering if it would make sense to form it as the following or are they effectively similar?

No creo que lo hubiera hecho mejor.

It seems that the subjunctive is warranted simply because "no creo" but I struggle with these sorts of constructions. Am I nitpicking or is this slightly off?

Answer 1

Your first sentence is not formed correctly. Note that the positive, indicative form is:

Creo que hice lo mejor que podía hacer.

When you change it to negative form, what needs to be in subjunctive is the main verb of the proposition you now distrust, which in this case is hice. You need to use the present perfect subjunctive, which is haya hecho. The subordinate should stay in indicative:

No creo que haya hecho lo mejor que podía hacer.

Although, as your sentence is in first person, it's better to use the infinitive:

No creo haber hecho lo mejor que podía hacer.

About your second sentence, its main problem is that it conveys a meaning which is in some sense opposite to that of the first. Your first sentence means:

I don't think I did the best I could.

And the second:

I don't think I would have done it better.