"I put the pickles on a small plate."
Translation:J'ai mis les cornichons dans une petite assiette.
"Sur" is possible as well, since we don't know if the plate is flat or not.
Putting food in a plate sounds really odd to me.
In English it you would put food
on a plate (which is mainly flat) and
in a bowl (which is deeper, e.g. for porridge, like in the story of Goldilocks and the three bears).
Why does this have to be past tense? I put can be present tense. I did put or I have put would be past tense.
What you say makes sense. It all depends on context.
"I put" is both the present and the simple past tense (for an action that took place at a definite time in the past).
We do not know the context, but it could be...
"I put the pickles on a small plate this morning." You can omit the "this morning" and it can still be understood as being in the past if you are not doing it then and there.
For the present tense, the context could be... "Look at how I am doing this. I put the pickles on a small plate." Said whilst putting them on the plate. Maybe this usage comes up less often (unless you teach cookery).
Cornichons are gherkins, not pickles in general. According to the dictionary the French for 'pickles' is 'pickles'!