- Every single day when I sit to write something, I am to think what I will write.
- Every single day when I sit to write something, I am to think what I would write.
Please say what are the differences between 1 and 2?
The questions themselves are not grammatically correct, and the I am to think bit is rather confusing, but I get your meaning, so I am going to focus purely on will and would.
I think to myself: what will I write?
Will has many meanings, with the words placed around it filling in the gaps. In the above sense, the implication is that something is going to be written one way or the other. It really does not matter what is written, only that the act of the writing is done.
I think to myself: what would I write?
Would can easily make the above hypothetical. If, every single day, I sat down to write something, I think to myself, what would I write? Here, the I think to myself is practically parenthetic:
If, every single day, I sat down to write something (I think to myself), what would I write?
In this sense, will speaks to actually getting some writing done, whereas would implies thinking about it without actually starting.
Will = action. Would = thought.
This is not a pure definition of either word, in and of themselves - it is how they fit in the puzzle that is your question.