"Pick up something" or "pick something up"?

Question

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]

Answer 1

This from the ‘Longman Student Grammar of Spoken and Written English’:

Where the direct object is a pronoun (1), it is usually placed between the verb and the particle (over 90 per cent of the time) . . . However, when the direct object is an indefinite pronoun (2), it is often placed after the adverbial particle . . . When the direct object is a full noun (3), there is more variation in its placement.

For Example:

1. Pick it up.
2. Pick up something.
3. Pick the pencil up. Pick up the pencil.

Answer 2

If "something" is short, you may use either one. But if it is long, put the "up" just after the "pick".

Pick up from the floor all the pencils, books, dolls, and blocks.

more easily understood than

Pick all the pencils, books, dolls, and blocks up from the floor.

Answer 3

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.