The Pascal used is fairly similar to C - I suspect you could understand it if you gave it a shot.
There is a Pascal to C conversion in effect for the HTML5 cross compile, but it makes the code a lot less readable, since the C has no function or operator overloading, or units.

Compare:
http://m8y.org/tmp/uGears.c
and
https://code.google.com/p/hedgewars/source/browse/hedgewars/uGears.pas?r=cffa2c8275e0
https://code.google.com/p/hedgewars/source/browse/hedgewars/GSHandlers.inc?r=cffa2c8275e0

The use of hwFloat math makes the C particularly tough. Especially if there's a fair amount of it, since uFloat uses operator and function overloading quite a lot.
Compare the Pascal:
resetdy:= resetdy + hwPow(resetdy,3)/_6 + _3 * hwPow(resetdy,5) / _40 + _5 * hwPow(resetdy,7) / resety + resetx * hwPow(resetdy,9) / resetdx;
versus the C:
resetdy = ufloat_hwFloat_hwFloat_op_add(ufloat_hwFloat_hwFloat_op_add(ufloat_hwFloat_hwFloat_op_add(ufloat_hwFloat_hwFloat_op_add(resetdy, ufloat_hwFloat_hwFloat_op_div(ufloat_hwPow(resetdy, 3), _6)), ufloat_hwFloat_hwFloat_op_div(ufloat_hwFloat_hwFloat_op_mul(_3, ufloat_hwPow(resetdy, 5)), _40)), ufloat_hwFloat_hwFloat_op_div(ufloat_hwFloat_hwFloat_op_mul(_5, ufloat_hwPow(resetdy, 7)), resety)), ufloat_hwFloat_hwFloat_op_div(ufloat_hwFloat_hwFloat_op_mul(resetx, ufloat_hwPow(resetdy, 9)), resetdx));

But anyway, really, if you know C, the Pascal and C are not that different…
…Pascal
^.
…C
-> (well actually the Pascal ^ and . is a bit more consistent than C -> and . but anyway)

…Pascal
begin/end
…C
{ }

…Pascal
not/and/or
…C
!/&/|

Feel free to drop by IRC (live chat) if there's anything particularly confusing.

On to the questions.
Most missiles check collision here:
https://code.google.com/p/hedgewars/source/browse/hedgewars/uGearsHandlersMess.pas#318
See the "TestCollision" lines which come from:
https://code.google.com/p/hedgewars/source/browse/hedgewars/uCollisions.pas

With regards to AI, consider the bazooka:
https://code.google.com/p/hedgewars/source/browse/hedgewars/uAIAmmoTests.pas#134

It is generating trajectories, then, on impact with something solid, using the rate explosion code:
https://code.google.com/p/hedgewars/source/browse/hedgewars/uAIMisc.pas#477

to work out how many allies and enemies are injured, and weight it accordingly.

learn some basic pacal and back.
the code
https://code.google.com/p/hedgewars/source/browse/hedgewars/uAIAmmoTests.pas#156

what does it mean
if not (r > 1) then
r is square of the third edge and should be greater than 1,and not means skip this condition until r <= 1?why?guess r is very big,so will it never be less than 1?