A mentőmellény külső oldala vízálló.

In Hungarian, the verb van (to be) is normally left out in the present tense, third person when it links a subject to a noun or adjective.

So:

• A mentőmellény külső oldala vízálló. = The life jacket’s outer side is waterproof.

But in other persons, Hungarian does use the verb:

• Én fáradt vagyok. = I am tired.
• Te fáradt vagy. = You are tired.

So here, the missing is is completely normal.

Because oldala means its side or the side of it.

The base noun is:

• oldal = side

With the possessive ending:

• oldala = its side

In this sentence, Hungarian expresses the outer side of the life jacket by marking the possessed thing (side) rather than using a separate word like English of.

So:

• a mentőmellény külső oldala = the life jacket’s outer side / the outer side of the life jacket

Hungarian often shows possession mainly on the possessed noun, not necessarily on the possessor.

Here:

• mentőmellény = the possessor
• oldala = the possessed noun, marked with -a

So the structure is:

• a mentőmellény külső oldala
  literally something like the life jacket outer side-its

This is a very normal Hungarian possessive pattern.

Hungarian can also use a fuller version with -nak/-nek on the possessor:

• A mentőmellénynek a külső oldala vízálló.

That is also correct, but the version without -nek is often more natural in simple statements like this.

Külső is an adjective meaning outer, external, or outside.

It describes oldala:

• külső oldal = outer side

So:

• a mentőmellény külső oldala = the outer side of the life jacket

A learner may confuse it with words like:

• kívül = outside (usually an adverb)
• külső = outer/external (an adjective)

Here you need the adjective, because it describes a noun.

Hungarian usually uses one definite article for the whole noun phrase.

So:

• A mentőmellény külső oldala = The life jacket’s outer side

Hungarian does not normally repeat the article before every word the way English learners might expect. The single a covers the whole phrase.

Yes, partly. Hungarian very often forms compounds, and vízálló is a standard adjective meaning waterproof.

It is built from:

• víz = water
• álló = literally something like resisting / standing against

Together:

• vízálló = waterproof

You should learn it as a fixed adjective.

Because it is the predicate adjective in the sentence.

The sentence structure is:

• A mentőmellény külső oldala = subject
• vízálló = predicate adjective

So the basic order is:

• Subject + predicate

This is a very neutral word order in Hungarian.

You could also move vízálló to the front for emphasis:

• Vízálló a mentőmellény külső oldala.

That would put more focus on waterproof.

Yes, that is a useful way to think about it.

Hungarian possession often works like this:

• possessor first
• possessed noun later, with a possessive ending

So:

• mentőmellény = life jacket
• külső oldala = its outer side

Put together:

• a mentőmellény külső oldala = the life jacket’s outer side

This structure can feel unusual to English speakers at first, but it is very common in Hungarian.

The difference is possession:

• oldal = side
• oldala = its side / his side / her side / the side of it

Examples:

• Ez egy oldal. = This is a side.
• A doboz oldala piros. = The side of the box is red.

In your sentence, oldala is necessary because it is the side of the life jacket, not just a side in general.

Hungarian third-person possessive endings can appear in different forms, such as -a/-e or -ja/-je, depending on the word.

For oldal, the correct form is:

• oldala

not:

• oldalja

This is just the standard possessive form of that noun, so it is best learned as a vocabulary pattern:

• oldal → oldala

Many nouns behave regularly, but whether a j appears is something learners often need to get used to case by case.

Yes, that is a natural English translation too.

The Hungarian phrase:

• a mentőmellény külső oldala

can correspond to several natural English expressions, such as:

• the life jacket’s outer side
• the outer side of the life jacket
• the outside of the life jacket

The exact English wording may vary, but the Hungarian structure stays the same.

Here it is in its basic dictionary form, with no visible case ending.

That may seem surprising, because in English we use of or ’s, but Hungarian can express possession without adding a special ending to the possessor.

So:

• mentőmellény = bare form
• oldala = marked as possessed

That is enough to show the relationship.

Yes, Hungarian word order is flexible, but changing it changes the focus or emphasis.

Neutral version:

• A mentőmellény külső oldala vízálló.

Emphasizing waterproof:

• Vízálló a mentőmellény külső oldala.

Both are grammatical, but the original sentence is the most straightforward, neutral way to say it.