A bilha guarda água fresca.

Here a means the.

In Portuguese, a is the feminine singular definite article:

• a bilha = the jug
• uma bilha = a jug

So A bilha guarda água fresca starts with The jug..., not A jug...

Bilha is a jug, pitcher, or earthenware vessel, often a traditional clay container used for water.

In Portugal, the word can suggest a more traditional container than a modern glass or plastic jug. Depending on context, words like jarra or cântaro might also appear, but bilha has a more specific, traditional feel.

Guarda is the 3rd person singular of the verb guardar.

Here it means something like:

• keeps
• stores
• holds
• preserves

So in this sentence, guarda does not mean only guards/protects in the literal sense. It means the jug keeps or stores fresh water.

Portuguese often leaves out the article with an uncountable noun when speaking in a general sense.

So:

• guarda água fresca = keeps/stores fresh water
• guarda a água fresca = keeps the fresh water or keeps the water fresh, depending on context

Without the article, água fresca sounds more general and less specific.

Because fresca agrees with água, which is a feminine noun.

In Portuguese, adjectives usually agree with the noun in gender and number:

• água fresca = fresh water
• vinho fresco = fresh/cool wine

So fresca is feminine singular to match água.

Yes. Água is definitely feminine.

That is why you say:

• a água
• água fresca
• esta água

Unlike Spanish, Portuguese does not switch to a masculine article here. So it stays feminine:

• correct: a água
• not correct: o água

Because in Portuguese, descriptive adjectives usually come after the noun.

So:

• água fresca = fresh water
• casa grande = big house
• livro interessante = interesting book

Sometimes adjectives can come before the noun, but that often changes the style, emphasis, or meaning. In this sentence, água fresca is the normal, neutral order.

A careful European Portuguese pronunciation is approximately:

a BILH-a GUAR-da Á-gua FRESH-ca

More closely:

• bilha: the lh sounds like the lli in million
• guarda: guar- sounds like gwar-
• água: two syllables, roughly A-gwa
• final a in European Portuguese is often reduced, so it sounds less open than in many other varieties

A rough learner-friendly version could be:

uh BEE-lyuh GWAR-duh AH-gwuh FREJ-kah

The exact sound of s in fresca can vary by accent, but in Portugal it often sounds like sh before certain consonants.

Yes, it is grammatical and understandable.

That said, depending on the exact meaning, a Portuguese speaker might also say:

• A bilha guarda água fresca. = The jug stores/keeps fresh water.
• A bilha mantém a água fresca. = The jug keeps the water cool/fresh.
• A bilha conserva a água fresca. = The jug preserves the water fresh/cool.

So the sentence is fine, but guardar focuses more on keeping/storing, while manter or conservar can sound more natural if the idea is specifically keeping the water cool.