Compare a melodia com a harmonia.

It is the affirmative imperative form used with você (and also o senhor / a senhora).

So Compare a melodia com a harmonia. is a command or instruction.

A useful comparison:

• você command: Compare
• tu command: Compara (standard form)

This is why compare may look like a present subjunctive form too: the você imperative uses that same shape.

Portuguese often leaves the subject pronoun out, especially in commands and instructions.

So instead of saying Você compare..., Portuguese normally just says Compare...

That sounds natural in:

• exercises
• manuals
• recipes
• classroom instructions

If you add você, it usually sounds more emphatic or marked.

Because melodia and harmonia are both feminine singular nouns.

So the correct definite article is a:

• a melodia
• a harmonia

This a means the, not to.

In normal Portuguese, you usually keep the article here.

So Compare a melodia com a harmonia sounds natural, while Compare melodia com harmonia sounds incomplete or telegraphic, like a note or heading.

Portuguese uses definite articles more often than English does with common nouns.

Because comparar A com B is the standard way to say compare A with B.

So:

• comparar a melodia com a harmonia

Here, com simply links the two things being compared.

Yes, that is possible.

Portuguese allows both:

• comparar A com B
• comparar A a B

But there is often a small nuance:

• comparar A com B = the most neutral everyday way to compare two things
• comparar A a B = can sound a bit more formal, literary, or like liken A to B

If you use a before a harmonia, it becomes à because of contraction:

• a + a = à

So:

• Compare a melodia à harmonia.

Because com normally does not contract with articles in modern standard Portuguese.

So you say:

• com a
• com o
• com as
• com os

That is different from contractions like:

• de + o = do
• em + a = na

The main special forms with com are with certain pronouns:

• comigo
• contigo
• consigo
• conosco

Not necessarily.

It is the command form associated with você, and in Brazilian Portuguese você is very common and often neutral, not especially formal.

So Compare... is perfectly normal in:

• school instructions
• written exercises
• neutral spoken Brazilian Portuguese

If someone uses tu, the expected standard command is usually Compara...

No. In Portuguese, h at the beginning of a word is usually silent.

So harmonia is pronounced basically like armonia.

That is true in many common words:

• hotel
• hoje
• homem
• harmonia

The sound comes from the following letters, not from the h itself.

Because their stress follows a normal Portuguese pattern, so no written accent is needed.

They are pronounced:

• meloDIA
• harmoNIA

In Portuguese, many words ending in -a, -e, or -o are naturally stressed on the penultimate syllable, so they often do not need an accent mark.

Yes. That is also correct.

Both are grammatical:

• Compare a melodia com a harmonia.
• Compare a harmonia com a melodia.

The difference is mostly in focus: the first noun is the one you present first as the starting point of the comparison.