Learn Hardcore Japanese: watasi ha asita tomodati ni renraku wo simasu.

In Japanese, は marks the topic of the sentence—the thing you’re talking about—whereas が often marks the subject, especially when introducing new information or emphasizing who does something. Here, 私 (I) is already the assumed doer of the action, so we use は to say “As for me, …”

What role does 明日 play, and why is it at the beginning?

明日 (tomorrow) is a time expression. Japanese sentences typically put time words early—often before the object—so you hear: “Tomorrow (明日), as for me (私は), to a friend (友達に), contact (連絡を) will do (します).”

Why is に used after 友達 instead of another particle?

に marks the indirect object or the target of an action. Here the action is “contact,” so 友達に means “to (my) friend.” If you used を, you’d be treating the friend as a direct object, which doesn’t fit “contact” in Japanese.

Why is を used after 連絡? Isn’t 連絡 a noun?

Yes, 連絡 is a noun meaning “contact/communication.” To turn it into a verb you pair it with する. In that compound, the noun takes を just like a direct object: 連絡をする = “to do contact” = “to contact.”

What’s the difference between 連絡をします and 連絡する?

連絡をします is the polite (ます) form; it’s what you’d say in everyday conversation to someone you don’t know well or in a formal situation. 連絡する is the plain/dictionary form, used in casual speech or writing, for instance with close friends or in notes.

Could I say 私が明日友達に連絡をします instead?

You can, but it changes nuance. Using が (私が) emphasizes “I” as the one who will do it, as if correcting someone else or answering “Who will contact your friend?” The は version just sets “I” as the topic, with no special emphasis.

How do you pronounce 明日 here, and are there alternatives?

Here it’s read あした (ashita). Another common reading is あす (asu), more formal or written. Both mean “tomorrow,” but あした is more conversational.

Does 連絡をする tell me how you’ll contact your friend?

No, it’s generic. 連絡をする covers phone calls, texts, emails, or any communication. If you want to specify, you’d say 電話をする (to call), メールをする (to email) or ラインする (to text on LINE), etc.