Learn Hardcore Hungarian: A csomag súlya fontos a postán. - The package’s weight is important at the post office.

Súly = weight.
Súly-a = his/her/its weight (3rd person singular possessive). In Hungarian, possession is usually marked on the possessed noun, not with a separate ’s.
So a csomag súlya literally works like the package its-weight → the package’s weight.

Why is it a csomag súlya and not something like a csomag’s súly?

Hungarian doesn’t use an English-style ’s. Instead, it typically uses:

the possessor as a normal noun: a csomag (the package)
the possessed noun with a possessive ending: súlya (its weight)

That’s the most common, neutral possessive structure.

Could I also say a csomagnak a súlya? What’s the difference?

Yes. A csomagnak a súlya fontos a postán. is grammatical.

a csomag súlya = more compact, very common
a csomagnak a súlya (with -nak/-nek) = more explicit/contrastive, can sound like you’re emphasizing whose weight it is (e.g., the package’s weight, not something else’s).

Both mean the same basic thing here.

Why is there no verb for is in the sentence?

In present tense, Hungarian usually omits van (to be) in sentences like X is Y:

A csomag súlya fontos. = The package’s weight is important. You would use van mainly when expressing existence/location or in some emphatic/other structures, but not in a simple present-tense “is + adjective” predicate.

What role does fontos play—adjective or adverb? Does it change form?

Fontos is an adjective meaning important, used predicatively here.
Hungarian adjectives generally do not change for gender/number the way they do in some languages. So fontos stays fontos.

Why is it a postán and not a posta or a postánál?

Postán is posta + -n (the superessive case), often translated as at/on. With places/institutions like posta (post office), Hungarian commonly uses this case to mean at the post office (i.e., there, in that context).
Postánál (-nál/-nél) is more like at/by next to (physically next to it), so it can sound more literal-location-focused than “at the post office (as an institution/service place).”

Why isn’t it a postában (like “in the post office”)?

Postában uses -ban/-ben (inessive) meaning in(side). It’s possible, but it emphasizes being physically inside the building.
A postán is the more idiomatic way to mean at the post office in the sense of dealing with postal services / at that place in general.

Why do we have a twice: A csomag … a postán?

Hungarian uses the definite article a/az similarly to English the.
Here, you’re talking about two definite things:

a csomag = the package
a postán = at the post office

Repeating the article is normal; each definite noun phrase typically has its own article.

When would it be az instead of a?

Az is used before words that start with a vowel sound (for easier pronunciation), otherwise a:

a csomag (starts with cs, a consonant sound)
a posta (starts with p, a consonant sound) But: az üzlet, az állomás, etc.

Is the word order fixed? Could I say A postán fontos a csomag súlya?

Yes, you can. Hungarian word order is flexible and often reflects emphasis/focus:

A csomag súlya fontos a postán. (neutral: statement about the package’s weight)
A postán fontos a csomag súlya. (focuses more on at the post office as the relevant context)

Both are correct; they just highlight different parts.

How do I pronounce csomag súlya? Anything tricky?

Key points:

cs is a single sound like ch in church.
súlya has a long ú (hold it longer): sú-.
ly in modern standard Hungarian is pronounced like y (as in yes), so súlya sounds roughly like SOO-yah (with Hungarian vowels).

If there were multiple packages, how would this change?

One natural plural version is:

A csomagok súlya fontos a postán. = The packages’ weight is important at the post office.

Here csomagok is plural (packages). súlya can still appear as a collective “their weight” in context, but you might also see more explicit plural-possessive forms depending on what exactly you mean (e.g., “each package’s weight” vs “the total weight”).