A inundação foi realmente grave.

Portuguese often uses the definite article more than English does. So a inundação means the flood or the flooding.

In a standalone sentence like this, a sounds natural because you are talking about a specific event that both speaker and listener can identify.

• A inundação foi realmente grave. = The flood was really serious.

If you removed the article, it would usually sound less natural in this context.

Yes, inundação is feminine, which is why it takes a:

• a inundação

A very common pattern in Portuguese is that nouns ending in -ção are feminine:

• a situação
• a informação
• a inundação

This is a very useful rule of thumb, although like most language patterns, you should still learn words individually.

Inundação means flood, flooding, or inundation.

It usually refers to an area being covered by water, often in a formal or neutral way. In everyday Portuguese, depending on context, you may also hear words like:

• cheia = high water / flood, often linked to rivers
• enchente = flooding, but this is much more common in Brazilian Portuguese than in Portugal

In European Portuguese, inundação is perfectly normal and often sounds slightly more formal or technical than some other options.

Foi is the 3rd person singular preterite of ser.

Here it means was:

• A inundação foi realmente grave. = The flood was really serious.

The preterite is used because the speaker is presenting the flood as a completed past event.

Yes. Foi can come from either:

• ser = to be
• ir = to go

So by itself, foi could mean either was or went.

But in this sentence, it must be from ser, because it is followed by the adjective grave:

• foi grave = was serious

If it meant went, the sentence structure would be different.

This is a very common question.

• foi gives a sense of a completed fact in the past
• era is more descriptive, backgrounded, or ongoing in the past

So:

• A inundação foi realmente grave. = The flood was really serious.
  This sounds like a complete statement about a finished event.

• A inundação era realmente grave.
  This could work in a larger context, but it sounds more like you are describing the situation while it was unfolding or giving background in a story.

In an isolated sentence, foi is the more natural choice.

Realmente means really, truly, or indeed.

In this sentence, it intensifies grave:

• realmente grave = really serious

It adds emphasis, similar to saying that the flood was not just serious, but genuinely or noticeably serious.

That position is very natural in Portuguese. The structure is:

• subject + verb + adverb + adjective

So:

• A inundação = subject
• foi = verb
• realmente = adverb
• grave = adjective

You could sometimes move adverbs around, but foi realmente grave is a normal, idiomatic order.

Not in the literal English sense of a grave in a cemetery.

Portuguese grave means:

• serious
• severe
• serious in extent or consequences

So here:

• grave = serious / severe

It is a very common adjective in Portuguese for serious situations:

• um problema grave = a serious problem
• uma doença grave = a serious illness
• uma inundação grave = a severe flood

Some Portuguese adjectives have the same form for masculine and feminine, and grave is one of them.

So you get:

• um problema grave
• uma inundação grave

The adjective stays grave in both cases.

It does change for number, though:

• problema grave → problemas graves
• inundação grave → inundações graves

Yes, absolutely.

• muito grave = very serious
• realmente grave = really serious / genuinely serious

They are similar, but not identical:

• muito focuses more on degree
• realmente can add a sense of confirmation or emphasis, like it really was serious

So both are correct, but they give a slightly different nuance.

A simple learner-friendly approximation would be:

• a inunda-SÃW foi real-MEN-te GRAV

A few useful pronunciation notes:

• inundação has nasal -ão, which is very common in Portuguese
• in European Portuguese, unstressed vowels are often reduced, so the sentence may sound more compressed than you expect
• grave is pronounced with a clear v sound at the end, though the final e is very weak in European Portuguese

If you are learning European Portuguese, listening repeatedly to native audio is especially important, because spelling and pronunciation often differ more than in many other languages.

It is mostly neutral, though inundação can sound a bit more formal or official than some everyday alternatives.

The whole sentence would fit well in:

• conversation
• news reports
• written descriptions
• official accounts

So it is not unusually formal, but it does sound slightly more standard and report-like than very casual speech.

A little, yes, but the original order is the most natural and straightforward.

Most natural:

• A inundação foi realmente grave.

Other possibilities may be grammatical, but they can sound marked or give a different emphasis. For example:

• Realmente, a inundação foi grave.
  This emphasizes realmente more strongly, like Really, the flood was serious or Indeed, the flood was serious.

For a learner, the original version is the best one to use.