Learn Hardcore Japanese: watasi ha rainen tomodati to ryokou ni ikitai desu. - I want to go on a trip with a friend next year.

In this sentence は marks 私 (“I”) as the topic of the conversation—what we’re talking about—rather than just the grammatical subject. If you used が, it would emphasize “I” as the one doing the action in contrast to someone else. With は, you’re simply stating something about yourself (“As for me, …”).

Why is 来年 placed before 友達と and the verb rather than at the end?

Temporal adverbs like 来年 (“next year”) typically appear early in Japanese sentences, often right after the topic marker. Japanese word order for modifiers (time, place, manner) is relatively flexible, but time expressions conventionally go near the front to set the scene before introducing other details.

What’s the function of と in 友達と?

The particle と here means “with.” It indicates companionship—“with a friend.” Whenever you want to say you’re doing something together with someone, you attach と to the person or people.

Why do we use に in 旅行に行く instead of を?

In 旅行に行く, 旅行 (“trip”) is treated more like a destination or waypoint, so it takes the directional particle に (“to”). In contrast, if you say 旅行をする, 旅行 is the direct object of “do a trip” and thus takes を. Both mean “to travel,” but the former emphasizes going somewhere for a trip.

How does 行きたい express “want to go”?

行きたい is the –たい form of 行く (“to go”). You take the verb stem 行き-, add たい, and it becomes an い-adjective meaning “want to go.”
Verb dictionary form: 行く → stem: 行き → add たい → 行きたい.

Why is です added after 行きたい?

Adding です after an い-adjective like 行きたい raises the politeness level.
Plain: 行きたい (“want to go,” casual)
Polite: 行きたいです (“want to go,” polite)

Can you omit 私 since the subject is obvious?

Yes. Japanese often drops pronouns when the context makes the subject clear.
Omitting 私:
来年友達と旅行に行きたいです。
This is perfectly natural and means the same thing. You include 私 only if you need to emphasize or clarify who wants to go.

Is the word order flexible? Could I say 私は友達と来年旅行に行きたいです?

Yes, to some extent. Japanese allows you to reorder most particles and modifiers (time, place, companion) as long as you don’t separate a noun from its particle. Your example:
私は友達と来年旅行に行きたいです。
is grammatically fine, though native speakers more often put the time word (来年) earlier. It still sounds natural and understandable.