Σήμερα το βράδυ μιλάμε χαλαρά στο σαλόνι.

In Greek, the present tense is very often used to talk about the near future, especially for plans and arrangements.

• Σήμερα το βράδυ μιλάμε χαλαρά στο σαλόνι.
  = Tonight we’re (going to be) chatting/We’re chatting tonight in the living room.

Using μιλάμε here suggests something like a planned / agreed activity, said in a casual, friendly way.

If you say:

• Σήμερα το βράδυ θα μιλήσουμε χαλαρά στο σαλόνι.

this is a bit more neutral or slightly more formal/decisive: we will talk tonight. Both are correct; the original just sounds more relaxed and conversational.

Greek does not form the present continuous with a separate verb like “to be” plus a participle.

The simple present form μιλάμε already means both:

• we talk
• we are talking

So:

• Μιλάμε. = We talk. / We are talking.

Adding είμαστε would be ungrammatical here. Greek expresses this continuous idea through the imperfective aspect of the verb (the present form), not by using an auxiliary verb like English does.

The subject pronoun εμείς (we) is usually dropped in Greek, because the verb ending already shows the subject.

• μιλάμε = we speak / we talk
  The -με ending tells you the subject is “we”.

You can say:

• Εμείς μιλάμε χαλαρά στο σαλόνι.

but that stresses “we” (as in: we are the ones talking, not somebody else). In neutral sentences, you normally just use μιλάμε without εμείς.

Σήμερα το βράδυ literally means “today the evening”, i.e. “this evening / tonight”.

• σήμερα = today
• το βράδυ = the evening / at night

Απόψε also means “tonight / this evening”. The differences:

• σήμερα το βράδυ
  
  • a bit more literal and slightly more neutral
  • very common in everyday speech
• απόψε
  
  • a bit more compact
  • may sound a little more literary or stylistic in some contexts, but it’s also used in normal speech

In this sentence you could say either:

• Σήμερα το βράδυ μιλάμε χαλαρά στο σαλόνι.
• Απόψε μιλάμε χαλαρά στο σαλόνι.

Both are correct and natural.

In modern Greek, time expressions with parts of the day often use the definite article, especially with σήμερα:

• σήμερα το πρωί – this morning
• σήμερα το μεσημέρι – this noon / this midday
• σήμερα το βράδυ – this evening / tonight

Saying σήμερα βράδυ is not standard; it sounds incomplete or foreign. You almost always want:

• σήμερα το βράδυ, αύριο το πρωί, χτες το βράδυ, etc.

There are also article-less adverbs like:

• βράδυ (as in το βράδυ = at night)
• αύριο (tomorrow), χτες (yesterday), σήμερα (today)

But in this specific pattern σήμερα + part of the day, keep the article: σήμερα το βράδυ.

Χαλαρά is an adverb, coming from the adjective χαλαρός / χαλαρή / χαλαρό (relaxed, loose).

In this sentence:

• μιλάμε χαλαρά ≈ we talk in a relaxed way / we chat casually / we talk chill

Nuances of χαλαρά:

• relaxed, not stressed
• easygoing, informal
• sometimes also: slowly, without rushing

Other examples:

• Πάρε το χαλαρά. – Take it easy.
• Το Σαββατοκύριακο το περνάμε χαλαρά. – We spend the weekend in a relaxed way.

So χαλαρά describes how you talk.

Μιλάμε and λέμε are related but not the same.

• μιλάμε (from μιλάω/μιλώ) = we speak / we talk
  Focus on the act of talking / having a conversation.
• λέμε (from λέω) = we say / we are saying
  Focus on the content of what is said.

In your sentence:

• Σήμερα το βράδυ μιλάμε χαλαρά στο σαλόνι.

means: Tonight we (will) have a relaxed chat in the living room.

If you said:

• Σήμερα το βράδυ λέμε χαλαρά στο σαλόνι.

this sounds strange or incomplete; λέμε usually needs what you are saying:

• Σήμερα το βράδυ λέμε τα νέα μας χαλαρά στο σαλόνι.
  Tonight we’re (going to) share our news in a relaxed way in the living room.

So for “chat / talk together”, μιλάμε is the natural verb.

Στο is a contraction of:

• σε (preposition: in, at, to)
• το (neuter singular definite article: the)

So:

• σε + το = στο

στο σαλόνι literally = in the living room.

Other similar contractions:

• σε + τον = στον (e.g. στον κήπο – in the garden)
• σε + την = στη(ν) (e.g. στη δουλειά – at work)
• σε + τους = στους (e.g. στους φίλους – to the friends)

Σαλόνι is a neuter noun. It usually corresponds to:

• living room, sitting room, lounge

Typical forms (singular):

• το σαλόνι – the living room (nominative/accusative)
• του σαλονιού – of the living room (genitive)

Examples:

• Το σαλόνι είναι μεγάλο. – The living room is big.
• Πάω στο σαλόνι. – I’m going to the living room.

In some regions or families you might also hear καθιστικό as another word for living room:

• στο καθιστικό – in the sitting room.

Yes. Greek word order is fairly flexible, especially with adverbs and time expressions. All of these can be correct:

• Σήμερα το βράδυ μιλάμε χαλαρά στο σαλόνι.
• Μιλάμε χαλαρά στο σαλόνι σήμερα το βράδυ.
• Σήμερα το βράδυ στο σαλόνι μιλάμε χαλαρά.

The default and most neutral here is the original: time → verb → manner → place. Moving elements around can slightly change the emphasis:

• Putting σήμερα το βράδυ first emphasizes when.
• Moving στο σαλόνι closer to the beginning could emphasize where more.

But all are understandable and natural in context.

Stress is important in Greek; it’s marked with the accent (´). Here’s where the stress falls:

• σήμερα – ΣΊ‑με‑ρα (stress on the first syllable)
• βράδυ – ΒΡΆ‑δυ (stress on the first syllable)
• μιλάμε – μη‑ΛΆ‑με (stress on the second syllable)
• χαλαρά – χα‑λα‑ΡΆ (stress on the last syllable)
• σαλόνι – σα‑ΛΌ‑νι (stress on the second syllable)

In fluent speech, the sentence flows something like:

ΣΊμερα το ΒΡΆδυ μηΛΆμε χαλαΡΆ στο σαΛΌνι.

The base verb appears in two forms:

• μιλάω
• μιλώ

Both are correct and widely used; they are just two variants of the same verb. They conjugate the same way in the present tense:

• (εγώ) μιλάω / μιλώ – I speak
• (εσύ) μιλάς – you speak
• (αυτός/αυτή/αυτό) μιλάει / μιλά – he/she/it speaks
• (εμείς) μιλάμε – we speak
• (εσείς) μιλάτε – you speak (pl./formal)
• (αυτοί/αυτές/αυτά) μιλάνε / μιλούν(ε) – they speak

Many learners find μιλάω easier because it looks more like the spoken form. For practical purposes, learn μιλάω as your “dictionary form”, and recognize that μιλώ is the shorter, equally correct variant.