Our Chevy Malibu has been checked by 3 different mechanics and nothing is wrong, yet our car continues to not turn on. What's wrong?

It started off on first cold front and
continued to be difficult to turn on
only when it was cold outside. But
then it got warmer and at first it
turned on fine then again it was
difficult. Since this first started to now
has been. San of one month. And
during that time on different
occasions we have had taken the car
to 3 different mechanics all saying
that nothing is wrong. Even so it won't
turn on unless we give it gas and that
doesn't work 100% all the time.
Despite having a good battery we
bought a new one. But nothing
changed. We cleaned spark blugs
then later changed them and still
nothing. Recently however we got a
check ingine light and found out it
was oxygen sensor. We asked one of
mechanics if this was cause of it not
starting and he said no and on top of
that we don't have to repair or change
oxygen sensor out. Next part we are
currently looking at is the fuel pump.
That's a $400 we don't have. But we
are lost and not sure what to do. I
have school and my fiance works we
need this car. Please, guide us... Is it
the fuel pump? Everything we tried
has not worked....