[i] I saw her clean the room.
[ii] He helped me do the work.
[iii] She made me clean the room.
What makes you think so?
Let me know what he said.
Bare infinitival has the meaning of ‘wholeness’ in [i] - I saw the whole event of her cleaning the room, not a segment of it as in ‘I saw her cleaning the room.' (CGEL, p.1237)
It now in [ii] has the meaning of ‘actual’ (direct) help - he actually did some of the work, whereas he ‘enabled me to do it myself' in ‘He helped me to do the work.’ (CGEL, p.1244)
About [iii], it's not easy to find out what their, the bare infinitivals', implications are. They seem to have the wholeness as in [i], but not connected with the actuality in [ii]. Are the bare infinitivals intending to say their actions are simultaneous with the main verbs’ activities, without delay – for ‘to’ is likely to project into the future?