Learn Hardcore Japanese: nimotu ga yoteidoori ni todoitara, sikkari naiyou wo kakuninsite oite. - If the package arrives as planned, be sure to check the contents carefully.

が marks the subject of the event inside the conditional clause 届いたら. It says “when the package arrives.”
は is a topic marker and is generally avoided inside conditional/time clauses like 〜たら. Using 荷物は here would sound odd or overly contrastive.
You could topicalize in the main clause (e.g., 荷物は予定通りに届きましたか), but inside 〜たら use が.

Why does 届いたら use the past form 届いた if it’s about the future?

The たら conditional is formed with the past (plain) form, but it doesn’t necessarily mean past time. 届いたら means “when/if it has arrived (at that point), …”
It often implies a sequence: first the arrival happens, then the action in the main clause.

Could I use と, ば, or なら instead of たら here?

届いたら: natural “when/if (once) it arrives,” good with instructions/requests.
届くと: “whenever it arrives, (inevitable result).” Not good with commands like 確認しておいて.
届けば: more hypothetical/conditional “if it arrives.” Possible, but less natural than たら for a one-time future event with a directive.
届くなら: “if it’s the case that it arrives / if you’re saying it will arrive.” Assumptive; context-dependent.

What exactly does 確認しておいて mean?

〜ておく means “do [something] in advance / and leave it done (for future convenience).”
確認しておいて = “go ahead and check (it) now so it’s taken care of.”
It emphasizes preparation and having the result in place for later.

How is 〜ておいて different from 〜てある?

〜ておく: speaker/doer’s intention to do something now for future benefit. Instructional here.
〜てある: describes a resultant state that exists because someone (unspecified) did it on purpose. Example: 内容はもう確認してあります (“the contents have already been checked [and are in that state]”).

Is ending with 〜ておいて a command? Should it be 〜ておいてください?

What nuance does しっかり add? How is it different from ちゃんと or きちんと?

Can I move しっかり after the object, like 内容をしっかり確認しておいて?

Yes. Both しっかり内容を確認しておいて and 内容をしっかり確認しておいて are natural. Placing しっかり right before the verb is very common.

Why is it 予定通りに and not just 予定通り?

予定通りに uses に to make it adverbial (“in accordance with the schedule / as scheduled”). Very standard.
In everyday speech, 予定通り届いた (dropping に) is also common and natural. With に can feel a bit more careful/formal.

Is 予定通り or 予定どおり correct? How do I read 通り here?

Both spellings are acceptable: 予定通り and 予定どおり. The reading is どおり.
Style guides often prefer kana (どおり) when it attaches to verbs/clauses (e.g., 言ったとおり), but after nouns both 通り and どおり are common.

Why 届く and not 着く for “arrive”?

届く focuses on something reaching the recipient (deliveries, mail, notifications). Very natural with 荷物.
着く is “to arrive (at a place)” and is used more for people/vehicles. For packages, 届く is the default. You’ll also see 到着する in formal/logistics contexts.

Is 内容 the best word here? What about 中身?

内容: the contents in terms of items/details/specifications (abstract listing). Common with 確認する.
中身: the physical stuff inside. With a package, 中身を確認する is also natural. Choose based on focus: specs/list (内容) vs physical items (中身).

Who is supposed to do 確認? There’s no “you.”

Japanese often omits pronouns. In an instruction like this, the subject is understood to be the listener (“you”). Context provides the actor.

Is this sentence polite or casual?

It’s casual/neutral directive as written (…確認しておいて。). In polite contexts, use …確認しておいてください。 Add softeners like ね/お願いします depending on relationship and tone.

What does the に in 予定通りに label here—time or manner?

It functions adverbially as “in accordance with / as per” the plan (manner-like). You can think “on schedule,” which is time-related in meaning, but grammatically it’s modifying how the arrival happens.

Can I emphasize immediacy, like “as soon as it arrives”?

Yes: 荷物が予定どおりに届いたら、すぐ内容をしっかり確認しておいて。 You can place すぐ(に) before the object or before the verb phrase.