Is this grammatically correct? "For one, I had mainly be"

Is the following grammatically correct?

The introduction of this assignment had made me rethink many previous ideas that I had held on to in previous assigned projects. For one, I had mainly been using Illustrator to...

Solution 1:

This is correct, as for one references back to ideas. See here:

People often say “for one” when they mean “for one thing”: “I really want to go to the movie. For one, Kevin Spacey is my favorite actor.” (One what?) The only time you should use “for one” by itself to give an example of something is when you have earlier mentioned a class to which the example belongs: “There are a lot of reasons I don’t want your old car. For one, there are squirrels living in the upholstery.” (One reason.)

Additionally, as I commented on the question, it would be

For one, I had mainly been...

Solution 2:

"I had mainly been" is fine, in the context you're using it. I might question whether or not the "mainly" adds any useful information.

However, in the first sentence, instead of "had" I would use "has". The introduction has, in the present, made you rethink previously held ideas. Saying "The introduction had made me rethink" seems to indicate you're saying something about a change of mind in the past. That may be what you meant, in which case I suggest taking out the "had" altogether -- "the introduction made me rethink".

The third "had" in those two sentences is OK, I think. However, I'd suggest removing the "on to", i.e. "rethink many previous ideas that I had held in previous assigned projects". Also, when you're saying that you are rethinking ideas, the context indicates that they are no longer ideas you hold now. So, you can remove the first "previous". "... rethink many ideas I had held in previous assigned projects."

Solution 3:

I would remove the first "had" - you mean to imply that you were using Illustrator until the introduction of the assignment, so using had in both cases fails to grammatically convey that point.

The introduction of this assignment made me rethink many previous ideas that I had held on to in previous assigned projects. For one, I had mainly been using Illustrator to...