Learn Hardcore Japanese: karui nimotu nara kaidan de hakobemasu. - If the luggage is light, I can carry it by the stairs.

と often expresses a natural or inevitable result (“when/whenever”), so 軽い荷物と階段で運べます would sound odd.
は would simply mark the topic (“as for light luggage”), not emphasize the condition. You could say 軽い荷物は階段で運べます, but that’s more like “speaking of light luggage, you can carry it on the stairs” rather than “if it’s light, you can carry it up.”

Why is there no を after 荷物 to mark it as the object of 運べます?

Here 荷物 belongs to the conditional clause (軽い荷物なら), so it can’t also take を. The structure is:

軽い荷物なら (conditional clause)
階段で運べます (main clause)
Since 荷物 is already inside the “if…” part, we don’t mark it again with を. If you wanted to mark a potential-verb object, you’d normally use が (e.g. 荷物が運べます), but in many everyday sentences you simply omit the object when it’s clear.

What role does で play in 階段で?

In 階段で, で marks the means or method (“by way of” or “using”). So 階段で運べます means “you can carry it using the stairs.” This is different from location-で (“at the stairs”) or the direct-object marker を (“pass through the stairs”), which wouldn’t make sense here.

Why is the verb 運べます used instead of 運びます?

運べます is the potential form of 運ぶ (“to carry”), meaning “can carry.” The base verb 運ぶ becomes 運べる in plain potential form, and the polite form is 運べます.

運びます is the simple polite form “carry,” without the nuance of ability.

Could we say 軽い荷物なら階段で運べる instead? How would it sound different?

Yes. 運べる is the plain (casual) potential form.

軽い荷物なら階段で運べる is perfectly grammatical in informal speech.
軽い荷物なら階段で運べます is the polite version, suitable for talking to customers or people you don’t know well.

Why isn’t there an explicit subject like “you” in this sentence?

Japanese often omits the subject when it’s clear from context. Here, the unspoken subject is a general “you,” “we,” or “people in general.” There’s no need to say あなたは or 私たちは because listeners naturally infer who can carry the light luggage.

Could we change 階段で to 階段を? What would happen?

No. 階段で marks the means (“by stairs”). If you use 階段を, you’d be marking “stairs” as a direct object—i.e. “to carry the stairs”—which doesn’t make sense. Always use で when you want to say “by means of” a route, tool, or method.

Is the word order fixed here? Could we say 階段で軽い荷物なら運べます?

Typical Japanese puts the conditional clause first (軽い荷物なら), then the place/method (階段で), and finally the verb (運べます). While you can shuffle elements for emphasis, 階段で軽い荷物なら運べます sounds awkward because it breaks the usual “condition → place/method → action” flow.