With verbs of observation: watch, see, hear, etc. the person (or thing) you are watching (or listening to, seeing, etc.) is an object and is followed by the main form of the verb--that is, the verb without the "to."
Watch her walk
See him swim
Listen to them sing
Hear him yell.
When you add the "as" you are not saying "watch her" but just "watch" as this other thing happens.
The woman walks.
The plural women walk. The form of the plural verb is the same as the main verb without the to.