Learn Hardcore Japanese: otouto ha mainiti zitensya de gakkou ni kayoimasu. - My younger brother goes to school by bicycle every day.

In this sentence, the speaker is talking about 弟 (おとうと – younger brother). In Japanese, 弟 by itself almost always means “my (the speaker’s) younger brother”. If you want to say someone else’s younger brother, you normally add their name or a pronoun:

私の弟 – my younger brother
田中さんの弟 – Mr. Tanaka’s younger brother
あなたの弟 – your younger brother

So 弟は毎日… naturally gets understood as “My younger brother …” without saying 私の every time.

What is the difference between 弟 and 弟さん?

Both mean “younger brother,” but they’re used for different people:

弟 – your own younger brother (or in-group younger brother)
弟さん – someone else’s younger brother (more polite/respectful)

So:

Talking about your own:
- 弟は毎日自転車で学校に通います。
  “My younger brother goes to school by bicycle every day.”
Talking about someone else’s:
- 田中さんの弟さんは毎日自転車で学校に通います。
  “Mr. Tanaka’s younger brother goes to school by bicycle every day.”

Using 弟さん for your own brother sounds oddly respectful toward your own family, which is usually avoided in Japanese.

Why do we use は after 弟 and not が?

は marks the topic of the sentence — what the sentence is about.
が usually marks the grammatical subject, especially when introducing new information or emphasizing who/what does the action.

Here, the sentence is about 弟 as a general topic: what he regularly does. That’s a classic use of は:

弟は毎日自転車で学校に通います。
“As for my younger brother, (he) goes to school by bicycle every day.”

If you used が:

弟が毎日自転車で学校に通います。

this would sound like you’re contrasting or emphasizing him specifically, for example:

(Others don’t, but) my younger brother is the one who goes to school by bicycle every day.

So は is natural here because it just sets up the topic.

Why is 通います used instead of 行きます for “goes to school”?

Both mean “to go,” but they’re used differently:

行きます (行く) – “to go” (one-time or simple movement)
通います (通う) – “to go to and from regularly,” “to commute,” “to attend (regularly)”

学校に通います means you regularly attend that school — it implies a repeated, habitual action.

Compare:

明日学校に行きます。 – “I will go to school tomorrow.” (a specific trip)
毎日学校に通います。 – “I go to school (commute to school) every day.” (regular attendance)

In your sentence, the idea is a daily routine, so 通います is the natural verb.

What does the particle で after 自転車 do? Why not another particle?

Here で marks the means or method by which the action is done — “by,” “with,” or “using.”

自転車で = “by bicycle,” “using a bicycle”

So:

自転車で学校に通います。
= “(He) commutes to school by bicycle.”

Other examples of で as “by / with”:

バスで行きます。 – I’ll go by bus.
ナイフで切ります。 – I cut it with a knife.
日本語で話します。 – I speak in Japanese.

So で is the correct particle to mark the method/vehicle of movement.

What does the particle に after 学校 do? Could we use へ instead?

Here に marks the destination of the action:

学校に通います – “commute to school / attend school.”

に can mark:

destination: 学校に行きます – go to school
location of existence: 学校にいます – be at school
time, purpose, etc. (in other contexts)

You can say:

学校へ通います。

へ also marks direction or destination, often translated as “to / toward.” The difference:

Xに行く – a bit more neutral/natural for “go to X (and be there)”
Xへ行く – slightly more directional “go toward X” (subtle nuance)

For everyday speech, 学校に通います is more common and sounds natural; 学校へ通います is also grammatically correct.

Why does 毎日 not have に after it? Can I say 毎日に?

Time expressions in Japanese split into two rough groups:

Specific points in time often take に
- ３時に行きます。 – I’ll go at 3 o’clock.
- 月曜日に会いましょう。 – Let’s meet on Monday.
Broad, regular, or relative times usually do not take に
- 毎日学校に行きます。 – I go to school every day.
- 毎朝コーヒーを飲みます。 – I drink coffee every morning.
- 来週行きます。 – I’ll go next week.

毎日 (“every day”) is in group 2, so you normally say:

毎日学校に通います。 (natural)
毎日に学校に通います。 (sounds wrong/unnatural in normal speech)

Can the word order be changed, like 弟は自転車で毎日学校に通います? How flexible is it?

Japanese word order is fairly flexible as long as the particles stay attached to the right words. All of these are grammatically possible:

弟は毎日自転車で学校に通います。
弟は自転車で毎日学校に通います。
弟は学校に毎日自転車で通います。 (less natural, but still understandable)

The most natural orders tend to put:

Time expressions like 毎日 fairly early.
Method (自転車で) before destination (学校に).

So your original:

弟は毎日自転車で学校に通います。

is very natural and easy to process. Changing the order can slightly affect emphasis or just sound less smooth, but it won’t usually break grammar as long as you keep は, で, に attached to the correct nouns.

What tense is 通います? Why does it mean “goes (regularly)” and not just “will go”?

通います is the non-past polite form of the verb 通う.

Japanese basically has:

non-past (dictionary form: 通う, polite: 通います)
past (dictionary: 通った, polite: 通いました)

The non-past form covers:

Present habitual:
- 弟は毎日学校に通います。
  – He goes / commutes to school every day.
Near future:
- 明日学校に行きます。
  – I will go to school tomorrow.

In your sentence, 毎日 (“every day”) clearly indicates a habit, so 通います is understood as “goes (regularly)” or “commutes,” not just “will go.”

How is 通います pronounced and what is the dictionary form?

通います is pronounced かよいます (kayoimasu).

Dictionary/plain form: 通う (かよう) – to commute, to go to and from regularly
Polite non-past: 通います (かよいます)
Polite past: 通いました (かよいました)
Te-form: 通って (かよって)
Plain past: 通った (かよった)

So if you make the sentence in plain (informal) style, you could say:

弟は毎日自転車で学校に通う。
– My younger brother goes to school by bicycle every day.