"I thought"

I wouldn't do it.

Usually it's unneeded. A first person narrative is pretty much all what the POV character thinks. Occasionally I'll use "I thought" if I'm pulling the focus from general description back to inside the POV character's head.

It depends. If you're just reporting the MC's thoughts in the moment, then I wouldn't. After all, in first person, everything is the MC's thoughts. But if the MC means something like "I believe," then "I thought" makes sense. "I thought he'd gone east but I wasn't sure."

What The IT says about marking a change of focus is good too.

If you're already in the character's POV, then you don't need to filter all the time, and filtering includes "he saw," "he heard," "he thought," etc.

You can format direct thoughts in italics:



Or you can use indirect thought, observed from the character's pov:



And in first person, you definitely don't have to use "I thought" -- it should be clear to the readers if something is reported as a thought from the narrator's POV. First person is already very much like a monologue or dialogue with the readers, so definitely no need to filter.



Of course, you can say "I thought" if you really want -- it's about voice sometimes:

If you need to define your character's thoughts, I would put them in italics (of course you could just keep them indirect). But, I don't believe you ever need to say "I thought".

d&f.

'I thought' for me would focus too much attention on the character and draw attention away from what he/she actually thought.

I've done a LOT of books in 1st person.

I've never used "I thought" unless it was:

I thought Joe would go along with the idea, but he said no.

When I want to show the character's immediate internal reaction, I use italics.

What an idiot. "Joe, we need to do this."

When you're writing first person, imagine that the protagonist is actually talking to the reader, telling this story. If the book is explicitly written as journal entries or letters, imagine it in that form.

Would this POV character, while telling their story, say/write "I thought?"

That's a question you'll have to address.

I'm doing first person on my current. I don't like using italics because it doesn't seem proper for my character, place, and time. Instead, the direct thought is embedded in the paragraph so it flows since basically everything is her thoughts. I treat it almost as a quote with "I think" as a "dialogue tag" but no quotation marks or paragraph breaks. Looking back at what I have so far in my WIP, I might need to get rid of a few of the "I thinks" in there.

On one occasion I put "I thought the house lacked life, but Charles told me that houses could not have life." I think that's another legitimate use, when your character is saying he/she had one opinion but other people had another.

BUT, like pretty much everything around here, it all depends on what your writing. Where/when is it? Who is your character? Maybe they're really insecure and always qualify everything. Maybe they just, well, think a lot.

I'm with TheIT and angeliz2k. It's often not needed, but there are times when it is--to indicate that a shift has taken place from what the character is observing/doing to what they're thinking, or perhaps if that's just the way they tend to narrate to themselves. As angeliz2k says, "Maybe they're really insecure and always qualify everything. Maybe they just, well, think a lot." I know I'm like that.

Depends on the sentence and if it's needed to redirect focus from external stuff to internal stuff.