Well, you see...
The one to the bottom left of the first T is S, which is obviously the preceding letter it in the alphabet. This is the same for the second T, and for the third letter (F), there's an E to the bottom-left of it.
All of these follow the pattern, so following that I got "O" for the blank spot. The hints weren't very helpful either - the final one was like:
As odd as it may seem, all the knowledge you need to solve this puzzle you probably had the day you stepped into first grade. It's safe to say that it's one of the first things you learned
And one of the first things I learnt was the alphabet, obviously.
Ugh.