Learn Hardcore Japanese: ato de sensei ni denwa wo simasu.

Reading: Ato de sensei ni denwa o shimasu. Natural writing without learner spaces: 後で先生に電話をします or more commonly 後で先生に電話します (drop を). All-hiragana: あとでせんせいにでんわをします.

Why is the teacher marked with に instead of を?

Japanese treats the person you call as the recipient/target, so it uses に: 先生に. Using 先生を with 電話する is ungrammatical. Parallel patterns: 人に会う (meet someone), 友だちにメールする (email a friend). Related: to say you received a call from the teacher, use から: 先生から電話がありました.

What is the role of を on 電話? Can I omit it?

電話 is a noun, and する turns it into a verb phrase (do a phone call): 電話をする. In everyday speech, speakers often drop を and say 電話する. Both are correct; the no-を version sounds a bit more natural in conversation: 後で先生に電話します.

Is 先生に電話をします natural, or should I always drop を?

It’s grammatical and fine, especially in careful or textbook-style speech. In day-to-day conversation, 先生に電話します is more common. Both forms are used.

Can I say 電話をかけます instead of 電話します? Any difference?

Yes. 先生に電話をかけます and 先生に電話します both mean you’ll call the teacher. 電話をかける literally means to place a call and can sound slightly more explicit; 電話する is shorter and very common. You still need 先生に, not 先生を.

What does で in 後で mean? Is it the same で as the place marker?

Here で adverbializes 後, forming the fixed time expression 後で (later/afterwards). It isn’t the location で. Think of 後で as a set adverb meaning later.

What’s the difference between 後で and 後に? When do I use each?

後で: common, conversational, often used on its own for general “later” (e.g., 後で電話します).
後に: more formal/written and typically follows a specific time/event (e.g., 授業が終わった後に先生に電話します, 昼食の後に電話します). Using bare 後に to mean “later” without specifying what it’s after is uncommon; use 後で.

Can I move 後で elsewhere in the sentence?

Yes. Time expressions are flexible:

後で先生に電話します (very common)
先生に後で電話します (also natural) Putting 後で right before the verb slightly emphasizes the timing.

Does します here mean future (will do)?

Japanese non-past (します) covers both present and future. The time word 後で supplies the future meaning, so the sentence means you will call later.

How polite is this? How can I make it more polite or humble for a teacher?

Polite: 後で先生に電話します or 後で先生にお電話します (adding お to 電話 is polite).
More humble/formal: 後ほど先生にお電話いたします or 先生にお電話差し上げます. Using お + verbal noun + します/いたします is a common polite/humble pattern.

Is 先生 the right word, and how do I include the teacher’s name?

先生 is a respectful title for teachers, doctors, lawyers, etc. With a name, say 山田先生. Addressing your own teacher directly, 先生 alone is fine. 教師 refers to the profession “teacher” and isn’t used as a title of address.

Can I use へ for the person, like 先生へ電話します?

No. For phone calls, standard Japanese uses に for the recipient: 先生に電話します. へ is for direction (e.g., 先生へ手紙), not for the person you call.

What’s the difference between 先生に電話する and 先生と電話する?

先生に電話する: to place a call to the teacher.
先生と電話する/電話で話す: to be on the phone talking with the teacher. Use と or say 先生と電話で話す when emphasizing the conversation.

Why is there no explicit subject like “I”?

Japanese often omits subjects when obvious from context. Here it’s naturally understood as “I.” If needed, include it: 私は後で先生に電話します.

Why are there spaces between the words here?

Standard Japanese doesn’t use spaces between words. The spacing is for learners. A natural version is 後で先生に電話します.

Is 後 read as あと or うしろ here?

Here it’s あと (later). うしろ is another reading of 後 meaning “behind,” which doesn’t fit this context. You may also hear the formal のち in 後ほど (later on).

How would I say “After class, I’ll call the teacher” using 〜てから?

授業が終わってから先生に電話します. This pattern means “after doing X, do Y.” It often alternates with V-た後に: 授業が終わった後に先生に電話します.