"Trotz" is usually used as "despite", and "trotzdem" is "nevertheless", but:
What is the grammatical difference?
Where does the verb go for "trotz" and where does it go for "trotzdem"?
Do Germans use either of these when they speak or is it one of the two that you only use when you write?
Do you use a comma with these?
trotz is a preposition like the English despite:
Example:
Trotz ihrer Schmerzen konnte sie den Marathonlauf beenden.
Since commas aren't needed with prepositions you needn't set one even if the word order changes like in this sentence:
Thomas feuerte den Mitarbeiter trotz dessen großer Verdienste für die Firma.
trotzdem can be used as adverb or as conjunction (same meaning as obwohl - not used in Standard German anymore).
Example as adverb:
Obwohl es ihm sein Vater verboten hatte, sah sich Timmy den Horrorfilm trotzdem an.
In the sentence above "trotzdem" is redundant; normally it's used in single sentences. Example:
Der Vater warnte seinen Sohn vor dem Film. Timmy sah sich den Horrorfilm trotzdem an.
Example conjunction
Er fuhr mit dem Auto, trotzdem er wusste, dass es kaputt war.
