Learn Hardcore Japanese: zikan wo kimetara, tomodati ni renrakusimasu. - Once I decide on the time, I’ll contact my friend.

The particle を marks the direct object of the verb. Here, 時間 (the time) is what you’re deciding on, so it takes を as in 時間を決める (“to decide the time”).

What does the たら ending in 決めたら mean?

～たら is a conditional form. It means “when/if … happens.” In 時間を決めたら, it conveys “once I decide the time” (or “when I’ve decided the time”), linking that condition to the next action.

Why is 友達 marked with に in 友達に連絡します instead of を?

With the verb 連絡する (“to contact”), the person you contact is treated as an indirect object and takes the particle に. You’re not “doing” something to the friend (direct object) but rather directing your contact toward them.

Why is 連絡します in the non-past (present) form when it refers to a future action?

Japanese doesn’t differentiate present and future tense with separate verb forms. The non-past form (連絡します) covers both “I contact” and “I will contact.” Context (like 決めたら) tells you it’s a future plan.

Can I use 時間を決めてから、友達に連絡します instead?

Yes. ～てから also means “after …,” so 時間を決めてから、友達に連絡します is perfectly natural. The nuance is nearly the same, though ～たら can feel a bit softer or more conversational than ～てから.

What’s the difference between ～たら and ～と for conditionals?

～たら: “when/if” with more emphasis on “once that happens.” It can describe a single event or a future plan.
～と: “whenever/if” but often implies a natural or automatic result. It’s less suited for one-off planned actions.
In this case, 時間を決めたら is better than 時間を決めると because you’re describing a planned sequence (“decide, then contact”) rather than a natural consequence.

Could I drop を in casual speech and say 時間決めたら?

In very casual speech (like texting), you might see 時間決めたら with particles omitted. However, in standard or polite Japanese you should include を for clarity: 時間を決めたら.