jeojjok geonmul ape taeksiga manhi seo isseoyo.

저쪽 literally means “that side over there” and emphasizes direction or area.
• 저 is a demonstrative adjective (“that”) used directly before a noun (e.g., 저 건물 = that building).
• 저기 is an adverb (“there”) indicating location but cannot modify a noun (you wouldn’t say 저기 건물).
So 저쪽 건물 highlights “the building on that side over there,” while 저 건물 is simply “that building.”

• -에 marks a static location where something exists.
• -에서 marks the place where an action originates or is in progress.
Since the taxis are simply standing (a static state) in front of the building, we use 앞에.

서 있어요 comes from 서다 (to stand) + 있다 (to exist). It indicates a standing or parked state.
• 있어요 alone would just mean “there are taxis.”
• 서 있어요 tells you the taxis are in a standing/parked position.

-가 highlights 택시 as the subject of existence/emphasis (“what is there?”).
Using -는 (택시는) would create a topic-comment nuance (“as for taxis…”), often implying contrast.

• 많이 is an adverb modifying the verb phrase (서 있어요), meaning “stand in large numbers.”
• 많은 is an adjective modifying 택시 directly (e.g., 많은 택시가 서 있어요).
Both are correct; 많이 서 있어요 is slightly more colloquial.

Yes. Korean word order is flexible with location phrases. For example:
• 저쪽 건물 앞에 택시가 많이 서 있어요.
• 택시가 저쪽 건물 앞에 많이 서 있어요.
Both convey the same meaning.

Change -있어요 to its past form -있었어요:
저쪽 건물 앞에 택시가 많이 서 있었어요.

• For a more formal/polite register, use -있습니다:
저쪽 건물 앞에 택시가 많이 서 있습니다.
• To add an honorific verb for animate subjects (not typical for taxis), replace 있다 with 계시다, yielding 서 계세요.