나는 더 피자가 먹고 싶어 ?
나는 더 피자가 먹고 싶어 I want to know where to place the "더"

how do you say "the other day"
what is 며칠 전에 and 일전에? are they used?

수고하셨습니다!? where is "셨" from?
difference between 수고하셨습니다 /수고했습니다 ?

Question

나는 더 피자가 먹고 싶어 ?
나는 더 피자가 먹고 싶어 I want to know where to place the "더"

how do you say "the other day" 
what is 며칠 전에  and 일전에? are they used?

수고하셨습니다!? where is "셨" from? 
difference between 수고하셨습니다 /수고했습니다 ?

Answer

Specifically, 셨 is 시+었 contracted. 시 is an honorific tidbit and 었 is part of the past tense conjugation.

Answer

1. 나는 피자가 더 먹고 싶어.
2. 'the other day' means 며칠 전에 or 일전에. Both are synonymous. However, '며칠 전에' is used more often. 
3. '수고하셨습니다' is more polite expression than '수고했습니다'