O lago é fundo.

O is the masculine singular definite article, equivalent to “the” in English.

• O lago = the lake (a specific lake that both speakers know about).
• In Portuguese, you normally cannot drop the article the way you sometimes can in English. You usually must say O lago, not just Lago, when you’re talking about a particular lake.
• For a non‑specific lake you would use the indefinite article: Um lago = a lake.

Both é and está mean “is”, but they come from different verbs:

• é = 3rd person singular of ser (used for more permanent or inherent characteristics)
• está = 3rd person singular of estar (used for temporary states, locations, changing conditions)

In O lago é fundo, being deep is treated as a permanent or defining characteristic of that lake, so you use ser:

• O lago é fundo. = The lake is deep (that’s the kind of lake it is).

Using O lago está fundo would sound unusual; you might only hear it in very specific, metaphorical, or joking contexts. For describing the normal depth of a lake, use é.

In Portuguese, most descriptive adjectives normally come after the noun:

• O lago fundo (or, more naturally, O lago é fundo) = The deep lake / The lake is deep.

Putting fundo before the noun (O fundo lago) is not standard in this meaning; it sounds wrong or at best poetic/odd.

Some adjectives can come before the noun (and sometimes change meaning when they do), but fundo as “deep” in this kind of sentence should follow the noun or the verb:

• O lago é fundo. ✅
• O lago fundo. (possible in some contexts, more like “the deep lake”, but less common as a standalone sentence)
• O fundo lago. ❌ (not used in normal speech)

Adjectives in Portuguese must agree in gender (masculine/feminine) and number (singular/plural) with the noun.

Base form (masculine singular): fundo

• Masculine singular:

  • O lago é fundo. – The lake is deep.

• Feminine singular:

  • A piscina é funda. – The swimming pool is deep.
  • A lagoa é funda. – The pond/lagoon is deep.

• Masculine plural:

  • Os lagos são fundos. – The lakes are deep.

• Feminine plural:

  • As piscinas são fundas. – The swimming pools are deep.

Pattern:

• -o → -a for feminine
• Add -s for plural: fundo, funda, fundos, fundas

Both can mean “deep”, and there is overlap, but there are tendencies:

• fundo

  • Very common, everyday word.
  • Used for physical depth: lakes, wells, holes, swimming pools, etc.
  • O lago é fundo. – The lake is deep.
  • Um poço muito fundo. – A very deep well.

• profundo

  • Slightly more formal or “literary”.
  • Also used for abstract or emotional depth:
    
    • um sono profundo – a deep sleep
    • tristeza profunda – deep sadness
    • silêncio profundo – deep silence
  • Can also be used for physical depth, but less colloquial.

In everyday speech about a lake, fundo is the most natural choice.

Yes, fundo can be both an adjective and a noun.

1. Adjective – “deep”

   • O lago é fundo. – The lake is deep.

2. Noun – “bottom”, “back”, “background”, “rear”

   • o fundo do lago – the bottom of the lake
   • o fundo da gaveta – the back/bottom of the drawer
   • no fundo da sala – at the back of the room
   • fundo musical – background music

So context tells you whether fundo is “deep” or “bottom/back/background”.

In European Portuguese:

• O – [u] (like the oo in “food”, but shorter)
• lago – LA-go:
  
  • la like “la” in “lava”
  • go like “go” but with a shorter o
• é – [ɛ] (like e in “bet”, but a bit more open)
• fundo – FUN-do:
  
  • fu roughly like English foon but with a shorter vowel
  • n is clear
  • do with a short o, closer to do in “dot” but less open

Word stress: LA-go, FUN-do (stress on the first syllable of each).

So roughly: [u 'la.gu ɛ 'fũ.du], with the un in fundo nasalized in typical European pronunciation.

Yes, lago is masculine, which is why we say o lago, not a lago.

A common rule of thumb (with exceptions):

• Nouns ending in -o are usually masculine:

  • o lago (lake)
  • o carro (car)
  • o livro (book)

• Nouns ending in -a are usually feminine:

  • a lagoa (pond/lagoon)
  • a casa (house)
  • a mesa (table)

There are exceptions (e.g. a mão, o mapa), so you gradually learn the gender with each noun. But for lago, masculine is standard: o lago é fundo.

For a normal full sentence, no. You need a form of ser or estar:

• O lago é fundo. – correct full sentence
• O lago fundo. – could appear as a noun phrase (“the deep lake”) in some contexts, but it is not a complete normal sentence in Portuguese.

So if you want to say “The lake is deep”, you must include é.

You can use intensifiers like muito or mesmo:

• O lago é muito fundo. – The lake is very deep.
• O lago é mesmo fundo. – The lake is really deep / It’s indeed deep. (European Portuguese usage)

You can also combine them for emphasis in speech:

• O lago é mesmo muito fundo. – The lake is really very deep.

You need to change the verb and the adjective for number and add não for negation.

• The lakes are deep.

  • Os lagos são fundos.
  • os = plural of o
  • são = plural of é
  • fundos = plural of fundo

• The lake is not deep.

  • O lago não é fundo.
  • não goes before the verb é.

Yes, if the context makes it clear you’re referring back to o lago:

• O lago é fundo. Ele é fundo. – The lake is deep. It is deep.

Notes:

• Ele is the masculine singular pronoun (“he / it”). Since lago is masculine, you use ele.
• In real conversations, you’d normally say one or the other, not both together.
• Using just É fundo. without ele would also be possible if the subject is very clear from context, but as a neutral full sentence with a pronoun you’d say Ele é fundo.

Both are correct but slightly different:

• O lago é fundo. – The lake is deep.

  • Refers to a specific lake both speakers know from context.

• Este lago é fundo. – This lake is deep.

  • este = this (near the speaker).
  • Often used when you are close to or pointing at the lake.

So o lago is “the lake” in general context; este lago is “this lake” in a more situational, deictic way.