Learn Finnish: Ei, juna ei ole vielä asemalla. - No, the train is not at the station yet.

The ending -lla is the adessive case, which generally means "on" or "at". While a station is a building, Finnish treats transit hubs like stations (asema) and bus stops (pysäkki) as open areas or points on a map. Therefore, you use -lla to say you are "at" the station.

Why is the word ei used twice here?

The first ei simply means "No". The second ei is the "it" form of the negative verb, which works together with ole to mean "is not".

Does vielä always mean "yet"?

It can mean "yet" or "still". In negative sentences like this one, ei ... vielä means "not ... yet". If the sentence were positive, like Juna on vielä asemalla, it would mean "The train is still at the station."

Why does the ending on asemalla use an "a" instead of an "ä"?

Because of vowel harmony. The root word asema contains the back vowel a. Even though it also contains the neutral vowel e, the back vowel dictates that the ending must also use a back vowel, making it -lla instead of -llä.