"Lucrând împreună cu voi, am învățat multe lucruri despre mașini."

Translation: Working with you I learned a lot of things about cars.

[AdrianKnot]

"Learned a lot about cars" is more idiomatic English.

[f_monts]

Why "working" and not "By working"? What's the difference?

[lcates55]

Looks similar to the ablative absolute from Latin.

[philtrew1]

a lot of things or many things? sounds the same to me