Terumot, Chapter One, Mishnah Ten
This mishnah completes the series of mishnayot that taught that terumah cannot be taken from one pile of produce that is one stage of processing on behalf of another pile that is in a different, either more advanced or less advanced, stage in its processing.
1) They may not take terumah from produce whose processing has been completed for produce whose processing has not been completed, or from produce whose processing has not been completed for produce whose processing has been completed or from produce whose processing has not been completed for other produce whose processing has not been completed.
2) If they did take terumah, their terumah is terumah.
Section one: Basically, the only time one can separate terumah from one pile of produce for another is if both piles are completely processed. For instance, if one has one barrel of wine, one could take terumah from it and thereby exempt another barrel of wine.
Section two: In all of these cases, if one nevertheless did take out terumah, the terumah is still valid.
This last clause seems to contradict that which was taught in the end of mishnah four, concerning one who gives terumah from olives or grapes in order to exempt oil or wine. One explanation for this is that that case was different because grapes and oil require a lot of work to process. Therefore, the rabbis penalized one who tried to give them to the priest instead of giving the oil or wine.
This clause also seems to contradict mishnah eight which said that the terumah is terumah, but he must go back and give terumah again. Again, one possible explanation is that there is a difference between oil and wine and other types of produce, and that the rules are stricter with regard to the former.