learning French, Why do i find it more difficult to "compile" a sentence than it is to comprehend a written one?

hey, join the club!

because to comprehend a sentence you can guess the meaning by looking at the written words whereas to compile a sentence you cannot guess, you must know the rules governing that language and the right word for that context, i.e. choosing from several synonyms.

linguists say that on average, to comprehend a text you need to know just 50% or the words you read.

Well if you are like me, you can't remember the start of the sentence by the time someone gets to the end of the sentence.

You have to interpret in real time, as the person is speaking.

You have to be faster, so you can "compile" something without missing the next word or phrase.

You can usually understand what it is, because for me especially because In french I forget to put à, le, etc. Somethings silly, but w/o them I can understand well.

Because reading is a receptive skill and writing is a productive skill. Receptive skills are easier and come before productive skills.