write checks one can't cash

as best one can

In the best possible way, given one's circumstances and abilities.

as far as one knows

To the best of one's knowledge, as far as someone knows or possible information to be gathered about the topic, subject, aspect, or situation.

back to square one

Located back at the start, as after a dead end or failure.

beat as one

To share the same feelings.

become one flesh

To join together in marriage; to develop a unifying bond as a result of marrying.

beggars can't be choosers

Something is better than nothing: those who can't provide for themselves should accept whatever help is available.

bite off more than one can chew

To try to do too much; to take on or attempt more than one is capable of doing.

bite the big one

To perform poorly; to fail.