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Questions & Answers about Lähdetkö aikaisin tiistaina?
How does the ending on tiistaina work?
The -na at the end of tiistaina means "on" a certain day. This ending is called the essive case. To say you do something "on Tuesday", you just take the word for Tuesday (tiistai) and add -na.
Why is there a -kö at the end of lähdetkö?
lähdet means "you leave". Adding the question particle -ko or -kö turns it into a yes/no question: "do you leave?" or "are you leaving?". We use -kö instead of -ko here because of vowel harmony (the vowels ä and e in lähdet pair with ö).
The dictionary form for "to leave" is lähteä. Why does it change to lähdet with a "d"?
This happens because of a rule called consonant gradation. When you add the -t ending for "you", it "closes" the syllable. This forces the strong t in the middle of lähteä to weaken into a d, making it lähdet.
I know aika means "time". Is aikaisin related to it?
Exactly! aika is the noun for "time", and aikaisin is the adverb meaning "early" (as in, ahead of time). You will often see Finnish vocabulary built from shorter root words like this.