Learn Hardcore Japanese: tyuusyazyou ha iriguti no tonari ni aru.

は marks the topic (“as for the parking lot…”), and the rest of the sentence is a comment about it. If you use が instead — 駐車場が入口の隣にある — you present it as new information or focus on what exactly is next to the entrance (“It’s the parking lot that is next to the entrance” / “There is a parking lot next to the entrance”). Use は when the parking lot is already known or being asked about; use が when introducing it.

Why do we use ある and not いる?

ある is used for the existence/location of inanimate things; いる is for animate beings (people, animals). A parking lot is inanimate, so ある is correct.

What is the に doing before ある? Why not で?

With existence verbs ある/いる, the location takes に: it marks where something exists. で marks the place where an action occurs. So:

Existence: 入口の隣にある (exists next to the entrance)
Action: 入口の隣で待つ (wait next to the entrance)

Why is there a の between 入口 and となり?

の links nouns. となり (隣) is a noun meaning “next to/neighboring.” The pattern X のとなり means “the place adjacent to X.” Other common patterns are 駅の前 (in front of the station), 学校の中 (inside the school).

Is となり the same as よこ, そば, or 近く?

They’re close but not identical.

となり (隣): adjacent/next to; often used for units in a sequence or of a similar kind (houses, rooms, seats, shops). Implies being right next to each other.
よこ (横): the side of something; immediate side position. Doesn’t require “same kind.” Example: ドアの横にポスターがある.
そば / 近く: “near/by” and “near/vicinity,” respectively; not necessarily adjacent. Example: 入口のそばに or 入口の近くに.

Can I write 隣 instead of となり? Any difference?

Yes. 隣 is the kanji for となり; meaning and usage are the same. In normal writing you’ll often see 入口の隣にある. The form 隣り (with the okurigana り) is an older style and uncommon today.

Could I say 駐車場は入口のとなりです instead of ある? What’s the difference?

Yes. Both are natural.

駐車場は入口の隣にあります: existential/locative (“The parking lot exists next to the entrance”).
駐車場は入口の隣です: equational (“The parking lot is next to the entrance”). In everyday speech, …隣です is very common. …隣にあります can sound a bit more factual or map-like.

What’s the polite version of the sentence?

With ある: 駐車場は入口の隣にあります。
With です: 駐車場は入口の隣です。

Can I change the word order? For example, 入口のとなりに駐車場があります?

Yes, and the nuance changes with topic/focus:

駐車場は入口の隣にあります。 Topic = parking lot; answers “Where is the parking lot?”
入口の隣に駐車場があります。 Presents new info “There is a parking lot next to the entrance.”
駐車場が入口の隣にあります。 Focuses on the subject “It’s the parking lot that’s next to the entrance,” e.g., in answer to “What is next to the entrance?”

Are the spaces in the original sentence normal?

No. Standard Japanese doesn’t put spaces between words. You’d normally write 駐車場は入口の隣にある。 The spaces were likely added for learners.

Is 入口 the same as 入り口? How do I read it?

Both 入口 and 入り口 are correct and read いりぐち (“iriguchi”). You’ll see 入口 often on signs; textbooks may use 入り口. Meaning and reading are the same.

What’s the difference between 入口 and 玄関?

入口: entrance to any place (store, building, parking area, etc.).
玄関: the entryway/foyer of a house or building (especially a home), where you typically remove shoes. For public facilities, use 入口.

What’s the difference between 駐車場 and パーキング?

駐車場 (ちゅうしゃじょう) is the standard word for “parking lot.” パーキング is a loanword used in names/signage and can sound a bit casual/commercial. In neutral speech, 駐車場 is safest.

Can I drop any particles here?

You can drop は if the topic is obvious from context, e.g., as a short answer: 入口の隣にある。 (spoken).
Don’t drop の or に in this structure; 入口の隣にある needs both to be grammatical.

How do I make this into a question or negative?

Polite question: 駐車場は入口の隣にありますか。 / or …隣ですか。
Casual question: 駐車場は入口の隣にある？
Polite negative: 駐車場は入口の隣にありません。 / …隣ではありません。 (also じゃありません)
Casual negative: 駐車場は入口の隣にない。