Compare a água doce com a água salgada.

Yes. In this sentence, Compare is an imperative form, so it means someone is being told or asked to do something: Compare ...

In Brazilian Portuguese, this form is commonly used for commands directed to você:

• Compare ... = Compare ... / Compare it ...

So the sentence sounds like an instruction, for example in a textbook, worksheet, or science activity.

Because Brazilian Portuguese usually gives commands to você using the present subjunctive form, which here is compare.

So:

• compare = command for você
• compara = command for tu

Since você is much more common in most of Brazil, compare is the form learners will see a lot in instructions.

That a is the definite article, meaning the.

So:

• a água doce = the fresh water

Portuguese uses articles more often than English does, especially with general nouns. Even when English might just say fresh water, Portuguese often prefers a água doce in a complete sentence like this.

For the same reason: it is also the definite article the.

So the structure is:

• Compare
  • a água doce
    • com
      • a água salgada

Portuguese often repeats the article before each noun phrase:

• the fresh water
• the salt water

This is very natural in Portuguese.

Because in Portuguese, adjectives usually come after the noun.

So:

• água doce = literally water fresh/sweet
• água salgada = literally water salty

This word order is normal in Portuguese. English usually puts the adjective first, but Portuguese often puts it after the noun.

Because adjectives change in different ways depending on their endings.

• doce ends in -e, so it usually has the same form for masculine and feminine nouns:
  • doce
• salgado / salgada changes according to gender:
  • masculine: salgado
  • feminine: salgada

Since água is feminine, we get:

• água salgada

And since doce does not change here, we simply get:

• água doce

Yes. Água is a feminine noun.

That is why you see:

• a água
• água salgada

The article a and the adjective salgada both show feminine agreement.

Literally, yes: doce often means sweet. But in this expression, água doce means fresh water.

This is a fixed, very common expression in Portuguese:

• água doce = freshwater
• água salgada = salt water

So you should learn água doce as a vocabulary chunk, not as water that tastes sugary.

Because comparar is very commonly used with com to mean compare ... with ...

So:

• comparar X com Y = to compare X with Y

In this sentence:

• Compare a água doce com a água salgada. = Compare fresh water with salt water.

This is the safest and most natural pattern for learners to use.

Yes, sometimes. Portuguese can also use comparar X a Y.

Very roughly:

• comparar X com Y = compare X with Y
• comparar X a Y = compare X to Y / liken X to Y

But in many everyday learning contexts, comparar ... com ... is the most straightforward choice.

You might be understood, but it sounds less natural in most normal Portuguese.

Portuguese usually prefers:

• Compare a água doce com a água salgada.

Leaving out the articles can sound more like a headline, label, or very compressed note. For a regular sentence, keeping a before both noun phrases is better.

A rough English-friendly approximation is AH-gwah.

More exactly:

• á = stressed AH
• gua = something like gwah, but the final vowel is lighter in Brazilian Portuguese

So:

• água ≈ AH-gwah

The stress is on the first syllable: Á-gua.

Because com normally stays separate from a in modern standard Portuguese:

• com a água salgada

Unlike some other prepositions, it usually does not form a standard contraction here the way:

• de + a = da
• em + a = na

So com a is the normal form.