Learn Hardcore Japanese: tegami ha poketto ni arimasu.

は is the topic marker, showing that “as for the letter” is what you’re talking about. If you used が, you’d be simply identifying or emphasizing the letter itself (“There is a letter”). Using は lets you focus on its location in the next part of the sentence.

Why is ポケット written in katakana?

Because ポケット is a loanword (gairaigo) from English pocket. Japanese typically writes foreign loanwords in katakana to distinguish them from native Japanese or Chinese-derived words.

What does the particle に do in ポケットにあります?

に marks the location where something exists. When you use verbs like あります (to exist for inanimates) or います (to exist for animates), you always pair them with に to show “at” or “in” that place.

Why use あります instead of います?

Use あります for inanimate objects (like letters, books, cars) and います for animate beings (people, animals). Since 手紙 (letter) is inanimate, you pick あります.

The sentence ends with あります—is that the polite form?

Yes. ある is the plain dictionary form meaning “to exist,” and あります is its polite present form. In casual speech you could say 手紙はポケットにある, but in polite conversation you say あります.

Could I say ポケットに手紙はあります?

Yes, that word order is grammatically possible because Japanese is relatively flexible. But by putting 手紙は first, you set the letter as the topic right away, which sounds more natural when you’re answering a question like “Where’s the letter?”

Why is the verb at the end of the sentence? It feels backwards compared to English.

Japanese follows a Subject/Topic – Object – Verb (SOV) order. The verb always comes last, so you state your topic (手紙は), then location (ポケットに), and finally the verb of existence (あります).