3. I ought to do some volunteer work.
4. You ought to let them do the talking. It's similar to "should"
USE: Ought to: has a similar meaning to should, but many of its uses have a sense of moral obligation (e.g., I ought to do some volunteer work.). Like Should, it can also have the meaning of “It’s a good idea to …” (e.g., You ought to let them do the talking.), as well as “This is probable or expected” (e.g., That ought to work.)