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Could someone please comment on my choices of prepositions? (correct / wrong / both are OK but X is more natural etc... Emotion: smile)

1) Throw it in / into the waste basket!
2) She put some butter in / into the pan.
3) He tucked his T-shirt in / into his trousers.
4) She put the book in / into her bag.
5) She put her hands in / into her pockets.
6) She shoved her hands in / into her pockets.
7) Insert the CD in / into the drive.
8) The soldier jumped in / into the trench.

1) into
2) both but I'd use into
3) into
4) both but I'd use into
5) both but I'd use into
6) into
7) both but I'd use into
8) both but I'd use into; Also, I think that in can be ambiguous here:

The soldier jumped in / into the trench and bayonetted the enemy machine gunner. BUT The soldier jumped inthe trench with joy. The war was over. Emotion: smile
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in is a normal and common substitute for into. It can be used in all your examples, so both in and into are correct in all eight cases. If there is a chance for ambiguity, of course you would want to use the full form into.

The reverse substitution is not possible, however. That is, you can't say *The chocolates are into the box.

CJ
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You may want to have a look at the discussion here:
http://tinyurl.com/nmqa8