Mixing past tenses in the same sentence

I have a question about violating verb tenses. I was taught that you can't use present perfect (or continuous) and past simple within a sentence. You either have to use present perfects (or continuous) with present perfects (or continuous), and past simples with past simples. But I've found this not to be true, so I'm a bit confused.

Here is an email one of my native American friends sent me.

Hey, it's been such a long time since we last emailed each other. Thanks for sending me an email! Getting that email was such a pleasant surprise, because I was just thinking how I've been wanting to send you an email as well.

What confuses me the most is the last part. How can you say that you were thinking about how you have been wanting to do something?

I was taught that this is wrong: "he has decided to go hiking, so I went hiking as well." But I'm assuming that my friend's email is correct, since he IS a native speaker.

So, this is what I'm thinking: I can't say present perfect (or continuous) + past simple, but I can say past simple + present perfect (or continuous).

This violates verb tenses:

"he has decided to go hiking, so I went hiking as well."

But this doesn't:

"I went hiking because he has decided to go hiking."

Are there instances where present perfect (or continuous) + past simple or vice versa is allowed and correct?

EDIT:

This is a difficult concept for me to grasp. But thank you for all your help, because I'm learning a lot. There are many sentences that don't make sense to me. I came across this sentence today:

"[I think this is inferred: During those days,] We've watched the movie many times. I really wanted this life to continue."

I'm not sure what the present perfect is doing here. I don't think it is being used for describing what a person has done before, like "I've seen that movie before". Why is it not simply "We watched the movie many times. [It was so good that] I really wanted this life to continue"?

Solution 1:

... you can't present perfect (or continuous) and past simple within a sentence.

As it stands, this rule is incorrect. In many cases it is acceptable and logical to mix past and present references in consecutive clauses

I lost my keys last week, but now I have found them.

This makes sense: A was true then, but B is true now.

This, however, does not make sense:

He has decided to go hiking, so I went hiking as well.

This sentence amounts to A was true then, because B is true now. The simple past describes a past event, your going hiking, but what the present perfect describes is not a past event, his decision, but a present state which is the result of a past event--his state of having decided. That present state cannot be the cause of the past event. The cause must be either a past event or a past state:

He decided (event) to go hiking, so I went hiking as well or
He had decided (state) to go hiking, so I went hiking as well.

The important thing is not to mix time references illogically.

As for the sentence in your friend's email:

Getting that email was such a pleasant surprise, because I was just thinking how I've been wanting to send you an email

There is no mixture of time references here, because the progressive construction "I have been wanting" marks a state, not an event, which may very reasonably be taken to continue into the present out of a past which is marked (by "just") as immediate. In effect, these pasts inhabit the same time frame as the present.

In any case, the "rules" are very loosely applied in informal discourse; see my discussion here. A casual email, which your friend probably dashed off in excitement, should not be held to the formal literary standards of coherence.

Solution 2:

The rule you quote is simplistic. It is quite permissible to mix all sorts of tenses within one sentence. The issue is whether the time relationships that they convey make sense.

Take a simpler case: present and future. Would a blanket rule that you should never mix present and future in a sentence be valid? No. "Bob owns the house and so he will paint it tomorrow." He owns the house in the present, and he will paint it in the future. Of course I can think of examples where mixing tenses would be nonsensical. "Bob got sick today from the fish he will eat tomorrow." Barring time travel, that's impossible.

The issue is not the physical order of the tenses, but the logical relationships of the events. As StoneyB notes, "He has decided to go hiking, so I went hiking as well," doesn't make sense because your past decision to go hiking cannot be dependent on "his" present continuous decision. This is not helped by simply switching the physical order to "I went hiking because he has decided to go hiking." The logical relationship of events still doesn't make sense. You could say, "He had decided to go hiking, so I went hiking as well," or "I went hiking because he had decided to go hiking." Now both events are in the past, presumably his decision came before yours. Or "He has decided to go hiking, so I will go hiking as well." Now "his" decision is in present continuous and your action is in the future, also plausible.

In short, don't think of it in terms of the ordering of clauses in the sentence. Think of it in terms of the order in which the events described occur in real life.