Two simple examples:
- a. I heard my mom cry.
b. I heard my mom crying.
- a. Please leave the door lock always.
b. Please leave the door locked always.
Which one, a or b, is right?
In (1), either is possible in principle, with a slight difference in emphasis. If you use the "-ing" form, you're focussing a bit more on the "mental image" of the action of crying; if you use the infinitive, you're stating it more as a "matter of fact" occurrence.
In (2), the only possibility of the two is "locked". If you use the infinitive with "leave", then you need to insert "to" as well: "I left him to do the washing up". If you use the causative "let" (don't confuse the meanings of these two verbs, which are slightly different), then no "to" is used: "I let him do the washing up".
N.B. In (2), the word order you have wouldn't be so natural, at least in UK English: one would tend to say "Please always leave...".