1.
po:532
auto:	⑤	DSN parameters
Original:	⌘0	Only if not exists

+ Only if it doesn't exist


2.
po:628
auto:	⑤	DSN parameters
Original:	⌘0	Only if exists

+ Only if it exists

OR, if you want to avoid using a pronoun:

+ Only if existing


3.
po:633
Original:	⌘0	If exists

+ If it exists
OR
+ If existing

as in #2.


4.
.po:742
Original:	⌘0	Inheritage

Unless this is an invented word, I think you want:

+ Inheritance

from Clytie, Vietnamese translator