I have a question in this sentence:
Ich wusste ja nicht, was mich da erwarten würde.
Why in the subordinate clause I have not used "Ich" instead of "mich"?
I do not get the meaning of the sentence, it doesn't make sense to me.
You can use "ich" and "mich" in that sentence with different meanings and they are exactly the same in english:
Ich wusste ja nicht, was mich da erwarten würde.
--> I didn't know what would expect me there.
Ich wusste ja nicht, was ich da erwarten würde.
--> I didn't know what I would expect there.
"Ich" is nominative case ("I"), "Mich" is accusative case ("me").
I think your confusion stems from the verb "erwarten". The primary translation (that you might have looked up in the dictionary) is "to expect". However, this meaning is slightly figurative. Literally, "erwarten" means "to await".
"Expect" works smoothly as a translation if a person is doing the "erwarten":
German: jemand (subject -> nominative) erwartet jemanden or etwas (both cases: object -> accusative).
English: someone (subject) expects/awaits someone or something (both cases: object).
The figurative meaning is almost too obvious to even notice that it's not exactly literal (you don't have to physically wait to expect something, after all).
However, in English, only someones can "expect", not somethings (because anticipation is an expression of sentience). So if you want to swap subject and object around (and you can, German is pretty liberal in that regard), "to expect" doesn't work anymore and you have to use the literal translation, which is "to await".
German: etwas (subject -> nominative) erwartet jemanden (object -> accusative).
English: something (subject) awaits (not: expects) someone (object).
This is what happens in your example. Unfortunately, "was" looks the same in both nominative and accusative case, so both variants are a bit difficult to tell apart. But "mich" has to be the object (it's in accusative form), and from that you can disambiguate the rest of the sentence.
