I have difficulties with word order:
- I have picked up the pencil from the floor. [says my dictionary]
?I have picked the pencil up from the floor. [could be?]
- ?I will pick up it. [sounds weird]
I will pick it up. [sounds right to me]
This from the ‘Longman Student Grammar of Spoken and Written English’:
Your first, second, and fourth examples are all good and resemble sentences people say all the time.
Number three, "I will pick up it", definitely sounds strange to me, though I really can't say why. If you replaced "it" with something specific, like, "I will pick up the pencil", it would be considered perfectly normal.