Learn Hardcore Japanese: watasi ha densya de gakkou ni ikimasu.

は is the topic marker. It attaches to 私 to indicate “as for me…,” so the full idea is “As for me, (I) go to school by train.” It doesn’t literally mean “subject” in the English sense, but rather what the sentence is about.

Why is は pronounced わ here, not は?

When は is used as a particle (specifically the topic marker), it’s always pronounced わ, even though the character looks like は. Inside words (like はな “flower”), it’s pronounced ha, but as a particle it becomes wa.

Why is で used after 電車? Doesn’t that mean “at” or “in”?

When で follows a noun of means or method (like 電車, バス, 自転車), it marks “by [that means]”—in other words, the method you use. So 電車で行きます means “go by train.”

What does に do after 学校?

に in this context is the destination marker. It tells you where you’re heading. So 学校に行きます is “go to school.”

Could I use へ instead of に for “to school”?

Yes—へ also marks direction (“toward”).

学校に行きます emphasizes arrival at school (“go and arrive at school”).
学校へ行きます emphasizes the direction (“head toward school”).
Both are correct and often interchangeable in everyday speech.

What is the dictionary form of 行きます, and why do we use –ます here?

The dictionary (plain) form is 行く (iku).

行きます is the polite present/future form you use when speaking politely to people you don’t know well, or in formal situations.
In casual speech with friends, you’d often say 行く instead of 行きます.

Do I have to include 私は every time, or can I drop it?

You can drop 私は whenever the topic (“me”) is clear from context. Japanese frequently omits the topic if it’s understood. So simply saying 電車で学校に行きます is perfectly natural once you know who you’re talking about.

Why are there spaces between the words in 私は電車で学校に行きます? I thought Japanese didn’t use spaces.

Good eye! Standard Japanese writing doesn’t use spaces. They’ve been added here just for learners to see the word boundaries. In real Japanese text you’d write it as 私は電車で学校に行きます.