O radiador está vazando.

O is the masculine singular definite article, equivalent to the.

So o radiador = the radiator.

In Portuguese, articles are used very often before nouns, sometimes more often than in English. In a normal full sentence, O radiador está vazando sounds natural and complete.

In this sentence, you can tell because it uses o, not a.

So:

• o radiador = masculine
• a radiadora would be feminine, but that is not the normal word here

Also, many Portuguese nouns ending in -or are masculine, though that is a pattern, not an absolute rule.

Portuguese has two main verbs for to be: ser and estar.

Here, estar is used because leaking is seen as a current condition or state, not a permanent characteristic.

So:

• está vazando = is leaking
• é vazando would be wrong

A good rule of thumb is:

• ser = identity, essence, general characteristics
• estar = current state, condition, location, ongoing situation

Vazando is the gerund form of the verb vazar, which means to leak.

So:

• vazar = infinitive
• vazando = leaking

In Brazilian Portuguese, estar + gerund is a very common way to express an action happening right now.

So está vazando works very much like English is leaking.

Both can be possible, but they do not emphasize exactly the same thing.

• O radiador está vazando strongly suggests an ongoing situation right now.
• O radiador vaza can mean the radiator leaks, the radiator tends to leak, or sometimes the radiator is leaking, depending on context.

For a problem happening at the moment, está vazando is the most natural choice in Brazilian Portuguese.

The accent in está shows the stressed syllable and is required in standard spelling.

It also helps distinguish está from esta:

• está = form of estar
• esta = this (feminine)

So the accent is not optional here.

In a normal full sentence, that sounds incomplete or less natural in standard Portuguese.

The usual form is:

• O radiador está vazando.

However, in certain informal contexts, you might see article omission in:

• notes
• headlines
• repair-shop shorthand
• text messages

For example, Radiador vazando could appear as a quick note meaning something like radiator leaking. But as a full spoken sentence, O radiador está vazando is the best standard version.

Not necessarily from grammar alone.

Radiador can refer to a radiator in general. In many real-life situations, people will assume a car radiator if that fits the context, but the sentence itself does not force that meaning.

Context tells you which radiator is meant.

People would probably understand it, but in Brazilian Portuguese that sounds unusual.

In Brazil, the natural structure is:

• estar + gerund
• está vazando

The form estar a + infinitive is much more associated with European Portuguese.

So for Brazilian Portuguese, O radiador está vazando is the form you should learn and use.

Very simply:

Question:

• O radiador está vazando?

Negative:

• O radiador não está vazando.

Portuguese often forms yes/no questions just by using the same word order and changing the intonation or adding a question mark in writing.

Yes. Radiador and radiator are clearly related words, which makes this an easy vocabulary item to remember.

Just be careful not to assume all cognates behave exactly the same way in grammar. Here, for example, Portuguese uses:

• an article: o radiador
• estar + gerund: está vazando

So the word is familiar, but the sentence structure is Portuguese.