Hodiaŭ estas malvarme, do mi portas mian mantelon.

Because malvarme is an adverb, and Esperanto usually uses an adverb in impersonal weather-style expressions:

• Estas varme. = It is warm.
• Estas malvarme. = It is cold.

English uses a dummy subject, it, but Esperanto does not need one. So the structure is just estas + adverb.

By contrast, malvarma is an adjective and would describe a noun:

• La vetero estas malvarma. = The weather is cold.
• La akvo estas malvarma. = The water is cold.

So in your sentence, estas malvarme is the normal way to say it is cold.

Esperanto does not use a meaningless subject like English it in expressions such as it is cold, it is raining, and similar cases.

So English:

• It is cold.

Esperanto:

• Estas malvarme.

This is completely normal. The verb can stand without an expressed subject in these impersonal expressions.

Hodiaŭ means today. It is an adverb of time.

In Hodiaŭ estas malvarme, it tells you when it is cold: Today, it is cold.

Esperanto word order is fairly flexible, so you could also say:

• Estas malvarme hodiaŭ.

But putting hodiaŭ first is very natural because it sets the time right away.

Hodiaŭ is pronounced roughly like ho-dee-ow.

The ŭ is a very short w-like sound. It appears in diphthongs, not as a full independent vowel. In aŭ, it sounds somewhat like the ow in now.

So:

• ho-di-aŭ

The stress in Esperanto is always on the second-to-last vowel sound group, so here the stress is on di:

• ho-DI-aŭ

Do means so, therefore, or thus.

It shows that the second part follows logically from the first:

• Hodiaŭ estas malvarme, do mi portas mian mantelon.
• Today it is cold, so I am wearing my coat.

It is a very common linking word in Esperanto.

Portas is the present tense of porti.

Esperanto verb endings are very regular:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command/jussive
• -i = infinitive

So:

• mi portas = I wear / I am wearing
• mi portis = I wore
• mi portos = I will wear

In this sentence, the present tense fits because the situation is true now: it is cold today, so I am wearing my coat.

It can mean both, depending on context.

• Mi portas sakon. = I am carrying a bag.
• Mi portas ĉapelon. = I am wearing a hat.

In your sentence, since the object is mian mantelon (my coat), the natural meaning is I am wearing my coat.

Because mian mantelon is the direct object of portas, and in Esperanto the direct object takes -n.

• Mi portas mantelon. = I am wearing a coat.

Also, adjectives agree with the nouns they describe in number and case. Since mia describes mantelo, it also gets -n:

• mia mantelo = my coat
• mian mantelon = my coat (as a direct object)

So both words change:

• noun: mantelo → mantelon
• adjective/possessive adjective: mia → mian

Yes. Words like mia, via, lia, ŝia, ĝia, nia, ilia behave like adjectives in Esperanto.

That means they agree with the noun they modify:

• mia mantelo = my coat
• miaj manteloj = my coats
• mian mantelon = my coat (object)
• miajn mantelojn = my coats (objects)

So mian has -n because it agrees with mantelon.

Mal- is a very common Esperanto prefix meaning the opposite of.

So:

• varma = warm
• malvarma = cold

And the adverbs are:

• varme = warmly / warm
• malvarme = coldly / cold

In weather expressions:

• Estas varme. = It is warm.
• Estas malvarme. = It is cold.

Learning mal- is extremely useful because Esperanto often forms opposites this way.

The comma separates the two clauses:

• Hodiaŭ estas malvarme
• do mi portas mian mantelon

This is similar to English punctuation before so in many sentences. It helps show the logical pause between cause and result.

In informal writing, punctuation can vary a little, but this comma is perfectly normal and clear.

Yes, to some extent. Esperanto word order is more flexible than English because endings show grammatical roles.

For example, these are all understandable:

• Hodiaŭ estas malvarme, do mi portas mian mantelon.
• Mi portas mian mantelon, ĉar hodiaŭ estas malvarme.
• Estas malvarme hodiaŭ, do mi portas mian mantelon.

However, the original sentence is very natural. It starts with the time expression, then gives the reason, then the result.

Normally, no. The usual word is simply hodiaŭ, which is an adverb meaning today.

Esperanto commonly uses adverbs of time directly:

• hodiaŭ = today
• morgaŭ = tomorrow
• hieraŭ = yesterday

So in this sentence, hodiaŭ is exactly the right form.

Yes, mantelo usually means coat.

So:

• Mi portas mian mantelon. = I am wearing my coat.

Depending on context, English might also translate it as cloak in older or more literary usage, but for everyday learning, coat is the best understanding here.

Yes, and that would specifically emphasize I am wearing my coat on my body.

• porti = to carry / wear
• surhavi = to have on, to be wearing

So:

• Mi portas mian mantelon. = I am wearing my coat.
• Mi surhavas mian mantelon. = I have my coat on.

In your sentence, portas is completely natural and common.