Which one of the following is correct?
- I was really thinking to do that.
- I really was thinking to do that.
I was really thinking to do that.
You might have assumed I was thinking about doing this, but actually that is what I have in mind (the word really confirms that as the option being considered).
I really was thinking to do that.
You might have assumed I had no real plans, but actually I was really thinking of doing that (the word really confirms the intention to do something).
In both cases, the word really implies the speaker thinks his audience may be labouring under a misapprehension (about the specific plan, or the fact of having any serious plan at all).
The more usual verb is think about, followed by the -ing form of the subsequent verb. Where you place really changes the emphasis. I really was thinking about doing that might follow a suggestion by someone else that you weren’t going to do it. I was really thinking about doing that is more neutral, and might occur unprompted. The meaning of both can be nuanced by the way in which they are pronounced.
I was really thinking to do that means that I wasn't thinking a little bit about doing that, nor somewhat: I was thinking a lot about doing it;
I really was thinking to do that means that it is true that I was thinking about doing that; you, or someone, said that I wasn't at all thinking about doing that, and that previous statement is not true.
Also, I prefer about doing that rather than to do that in both instances.