Before Jefferson left for France in 1784, he was an outspoken abolitionist, calling slavery "a perpetual exercise of the most boisterous passions" After Jefferson's return from France in 1789, he withdrew his outspoken opposition to slavery. What caused this change in behavior?

There's more to be said, but the great /u/freedmenspatrol has written a bunch of stuff about America's founding fathers and slavery, which often discusses Jefferson at length, including in these answers:

Did Jefferson and Washington desire slavery to be eradicated?
Were any of the founding fathers openly opposed to slavery?
Why didn't all of the early Presidents own slaves?
After the United States government adopted the ideal that all men have certain unalienable rights, how did we justify slavery and the atrocities committed against the Native Americans?