Learn Hardcore Japanese: asita sensei ni denwa wo simasu.

In this sentence, に marks the target or recipient of the action. Since you’re directing the phone call toward the teacher, 先生に電話をします literally means “I will phone the teacher.”

Why is 電話 marked with を?

Japanese turns many nouns into verbs by combining them with する. In the pattern Xをする, を marks X as the direct object of する. So 電話をする means “to do a phone call,” and 電話 needs を.

Why can’t we say 電話す instead of 電話をする?

電話 is a noun, not a verb. You need a verb to express the action “to phone.” Japanese uses する to verbalize many nouns. 電話す isn’t grammatical because す by itself isn’t a standalone verb for “phone.”

What’s the difference between 電話をする and 電話をかける?

Both mean “to make a phone call.”

電話をかける is a compound verb (かける) and very common when talking about dialing or placing a call.
電話をする is noun + する, slightly more generic and also correct. Usage often comes down to personal or regional preference.

Why is 明日 placed at the beginning without a particle?

Time expressions like 明日 function as adverbials and can appear at the start of a sentence without に. It sets the scene: “Tomorrow, …” You could say 明日に in very formal contexts, but everyday Japanese drops the particle for time words.

Which level of politeness is the verb form します?

します is the polite (–ます) form of する, so the sentence is in polite register. In casual speech you’d say 明日先生に電話する.

Why is there no subject in this sentence?

Japanese often omits the subject when it’s obvious from context. Here, it’s understood that “I” (or “we”) will make the call. You can add 私は for emphasis, but it’s not necessary.

Can we change the word order, for example 先生に明日電話をします?

Yes. Thanks to particles, Japanese allows flexible word order. Putting 先生に first is fine (先生に明日電話をします), though the most common flow is 明日 (time) → 先生に (target) → 電話を (object) → します (verb).