Learn Hardcore Japanese: ima ha isogasii desu kara, ato de denwa wo simasu.

It marks the topic: 今は means “as for now” or “right now (as opposed to other times).” It subtly contrasts the current time with other times when you might not be busy.

Can I drop は and just say 今忙しい?

Yes. 今忙しい is fine (casual). 今は忙しい adds a contrastive nuance (“now, at least, I’m busy”). In polite speech you’d say 今は忙しいです.

Why is です after 忙しい? Is it necessary?

忙しい is an i-adjective and can end a sentence by itself. Adding です makes it polite: 忙しいです. In the middle of a sentence, 忙しいですから is common in conversation; in casual speech you’d say 忙しいから.

How does から work here?

から attaches to the reason clause: 忙しいですから = “because (I’m) busy.” The result follows: 後で電話をします = “I’ll call later.” You can also split into two sentences: 今は忙しいです。後で電話をします。

What’s the difference between から and ので in this sentence?

Both mean “because.” から is a bit more direct; ので is softer/more objective: 今は忙しいので、後で電話をします。 Either is fine; choose based on how direct you want to sound.

Can I reverse the order and put the reason second?

Yes: 後で電話をします。今は忙しいですから。 That’s natural in speech. You can also start a new sentence with ですから to mean “therefore,” referring back to what you just said.

What is the で in 後で?

It’s the particle で forming the set adverbial 後で (“later/afterwards”). With a specific event, you say X の後で or X の後に (e.g., 会議の後で “after the meeting”). 後で alone means “later.”

Is 後で the same as あとで?

Yes, same word and pronunciation (atode). 後で (kanji) and あとで (hiragana) are both common. More formal alternatives are 後ほど or のちほど (“later”).

Why is を used with 電話? I often see 電話します without を.

電話 is a noun; 電話をする literally “to do a phone call.” In modern usage the を is often dropped: 電話します. Both 電話をします and 電話します are correct; with or without を there’s no real difference in meaning here.

Should I use 電話する or 電話をかける? Are they different?

Both mean “to call (on the phone).”

田中さんに電話します／電話をかけます = “I’ll call Mr. Tanaka.” The person you call takes に, not を.
電話をかける can feel a bit more literal (“to place/dial a call”), but in everyday speech they’re interchangeable.

How do I say “I’ll call you later” more explicitly?

Use the person + に:

（名前）さんに後で電話します。 Avoid あなた in most contexts; use the person’s name/title or omit the pronoun if it’s clear from context.

Does します here mean “will”? I thought Japanese has no future tense.

Japanese has a non-past form that covers present and future. 電話をします is interpreted as future here because of 後で (“later”) and the context.

Is the subject “I” missing? How would I add it?

Yes, it’s implied. You can add it if needed: 私は今は忙しいですから、後で電話をします。 Usually Japanese omits pronouns when context makes them clear.

Are the spaces and punctuation normal in Japanese?

Spaces are usually not used between words in Japanese. You’d typically write: 今は忙しいですから、後で電話をします。 The comma 、 marks a natural pause.

What are natural casual and more polite variants?

Casual: 今忙しいから、後で電話するね。
Polite: 今は忙しいので、後で電話します。
Very polite/business: ただ今取り込み中ですので、後ほどお電話いたします。

How do you read/pronounce the sentence?

今は忙しいですから、後で電話をします。
Reading: いまはいそがしいですから、あとででんわをします。
Romaji: ima wa isogashii desu kara, ato de denwa o shimasu.

Can I say 今が忙しい?

今が忙しい is possible but focuses on “now” as the thing that’s busy (strong emphasis/contrast) and is less natural here. To set “now” as a general topic, 今は忙しい is the default.