Learn English – Words that are their own past tense

This sounds much more natural to me than "I thought I surprised him."

Both have their place, and mean different things.

If the narrating perspective considers after the story (whether from an indistinct "now" of the storytelling, or from a particular framing time frame) that they had surprised "him", then this would be more appropriate.

If the narrating character thought then that they had surprised, then "I thought I surprised him" might be more appropriate. (The mixture of simple past and past perfect, "I thought I had surprised him" is different again, and also has its place).

Is there a word for this?

It's generally just called "mixing tenses" or "switching tenses".

It's important to note that while more basic instruction (whether to younger students, or to students of English as a second language) is not to mix tenses, this is in the category of "lies to children": Not only can one mix tenses in a great variety of ways, but one should mix tenses. The difference is between slipping from one tense into another (as perhaps might happen with a novice speaker more familiar with the present tense forms) and switching because it is appropriate to do so.

In your example of "I think I surprised him" is appropriate if present tense is appropriate for the thinking and past tense appropriate for the surprising, and not otherwise.

Really the rule is "always use a tense appropriate for what you want to express". Mixing tenses when one shouldn't is an easy way to break this rule for the most novice of English writers and speakers, but not mixing them when one should is another form of the same problem.

I sympathize with the heartfelt cry, "Does the English language not specify which tense that we must use when sentences are constructed like this?" The answer is sort of. "The English language" is really the speakers of the English language, and they don't always agree with each other, and they're not always consistent, especially in informal communication like the spoken dialog that you're reporting.

The first case you cite is, I think, the result of reported speech (or in this case, thinking.) If Mr. Bruce were reporting his thoughts directly, he would have told the narrator: I thought, "I will tell you when I see you." But Mr. Bruce tells the narrator a report of this thinking: I though I'd [I would] tell you when I saw you." This is called "backshifting the tense" for a past report ("thought"), and it takes "will" to "would," and "see" to "saw."

The second case has almost the same structure, but with a slight syntactic ambiguity about the reported thought. Is it of

    "I will tell you."

or is it of

    "I will tell you before you get to the office."

In the latter case, "get" should be backshifted to got. In the former, the temporal clause is not part of the report, and the verb "get" is in the present, which is used for near-future events.

Of course, that future sense clashes with the past "I thought," but getting the tenses technically correct would require something like, "Before you get to work, I will have thought that I had told you," and no one would say that.

There also is some sense of obligation in the sentence, not only that the speaker would warn the narrator but that he should. In which case, the sentence "I thought I should tell you before you get to work" is fine with the present tense indicating an ongoing situation.

Remember that it's dialog, and people don't always speak "correctly." Perhaps the author deliberately tried to mimic everyday speech, or perhaps he got it "wrong" in the first place.