Ah! I thought you were wondering how to listen to the word and know how to spell it, sorry about that. :)
So your question's easy: s by itself is never /sh/. At the beginning of a word like Spiel or Strand, st and sp is /sht/ and /shp/ respectively. In the middle or at the end of a word (not counting compound words like Schauspieler), they're pronounced separately and the s is just an /s/. Anywhere else, /sh/ is spelled out with sch.
Yes, precisely. A dictionary, too, confirms this syllable separation.
The reason Gestalt has an /sh/ sound is because "ge-" is a prefix. "Gestern" doesn't have a prefix, which I imagine is due to etymological reasons, but this is really rare in German, and not worth worrying about--better to just memorize the exceptions.
Prefix "ge-" is never stressed (stress falls on the next syllable) and of course "st" at the beginning of a syllable is /sh/, which is why you have the "ge-SCHTALT" pronunciation but "GES-tern".
There's no single past tense. "Was playing" is pluperfect, which is gespielt hatten. So "I was playing" is ich hatte gespielt. But that's only used if you're comparing something that was done before something else.
If the question is "Was hast du/er/sie gemacht?" (what were you/was he/was she doing?) then the answer is "Ich habe/er hat/sie hat gespielt." (We would say "I/he/she was playing" but it's literally "I/he/she played.") You can even say ich habe gespielt to this question and in context it means "I was playing." Ich spiele gestern Fußball is "I played soccer yesterday" and ich spiele morgen Fußball is "I will play football tomorrow." Those are used in everyday speech.
When speaking, "He played soccer" is er hat Fußball gespielt, and in a written story it's er spielte Fußball.
It's not altogether super complicated, but it does require some focused study. Usually in a German course you learn present tense, then simple and complex past tense, and then future tense, and then you loop back around to perfect and imperfect past tense, and so on. So unfortunately you won't get that from a Duolingo comment, but find a good book to accompany the course and you'll be better prepared. :)