pa - as prefix to many verbs

[stefandutoit]

When do you add 'pa' to a verb? eg. paprobavat'; pakushat' etc.

[toharsky]

When do you add 'pa' to a verb?

По- can be used not only with verbs, but also adjectives and adverbs.
The main cases for the verbs are
- when something is finished
пообедать (have dinner), познакомиться (make the acquaintance). it's somewhat of a sing that the verb is perfective
- when something continues for a period of time
потанцевать (to dance - for some time is implied), попутешествовать (to travel (for sometime))
- when something is started
побежать - to run (after someone for example)
- when something gains a certain quality
порозоветь (turn pink - about the sky before dawn or something like that)

The system doesn't seem to be overly logical though)
Hope I haven't forgotten too much))

Your examples are of perfective meaning, anyway. Попробовать also has this one-time action nuance.