Я подумал то что, я забыл много грамматических правил
Я думал, что забыл много грамматических правил
It's easy to explain. "То" is Ok, impulse just used it wrong with the perfective verb which needs an object. When you write "it's incorrect" in such cases, you confuse a student. I think it's not good.
"Я подумал о том, что..." is a good one, "о том" = "about that". Подумать о чем-то = To think about something.

Although in this context imperfective is preferable, because impulse used "but" at the next part of the sentence.

Some comments for impulse.

Look at this wrong phrase: "Я был не могут заняться русском языком..."
How did you check it? It's easy, if you try to use a reverse translation.
Ok, let's translate it: Я (I) был (was) не могут (can not, plural) заняться (practice) русским языком (Russian language).
We got an amazing English sentence: "I was can not practice Russian language."
At the moment it's easy to correct, we just need to replace horrible "was can not" with "could not" (= "не мог"): "I could not practice Russian language."
So, now Russian variant looks like this one: "Я не мог заниматься русским языком". Perfect!
This method is good enough for the simple sentences, if their grammatical structure is not complicated.

Also, the last comment about a style.
Let's look at the same sentence: Я не мог заниматься русским языком, потому что я был на отдыхе в Бодруме.
I underlined the second "я", because it sounds redundant. It's completely grammatical correct, but when you write in Russian, it's Ok to omit the second subject if it repeats the first one. We do it a lot (native English speakers can't afford the same, he-he) and this's good Russian. The verb "был" already conjugated with the first "я", because it's properly declined. So when a reader see the second part of your sentence, they can find a necessary subject ("Я") automatically. That's how the conjugate system works (the case system works in a similar way).
I hope it helps you to think "like a Russian."