Wir unterhalten uns lange.

Because dictionaries usually list the verb in its full reflexive form as sich unterhalten.

In an actual sentence, sich changes to match the subject:

• ich unterhalte mich
• du unterhältst dich
• wir unterhalten uns

So in Wir unterhalten uns lange, the uns is the reflexive pronoun that belongs with unterhalten.

Uns is the reflexive pronoun for wir.

So:

• wir = we
• uns = ourselves

In English, we usually would not say we entertain ourselves or we talk ourselves in this context, but German uses the reflexive structure with this verb. So wir unterhalten uns means we are talking / chatting / having a conversation.

It can mean different things depending on how it is used.

• jemanden unterhalten = to entertain someone
• sich unterhalten = to talk / converse / chat

So in this sentence, because it is reflexive (uns), it means to talk or to have a conversation, not to entertain.

In a normal German main clause, the conjugated verb usually comes in the second position.

Here:

• Wir = subject
• unterhalten = conjugated verb
• uns = reflexive pronoun
• lange = adverb

So the standard order is:

subject + conjugated verb + reflexive pronoun + other elements

That gives: Wir unterhalten uns lange.

Wir uns unterhalten lange is not correct standard word order for a main clause.

Good question. Sich unterhalten is not a separable verb in this meaning.

Compare:

• Ich unterhalte mich mit ihr. = I am talking with her.
• not Ich halte mich unter ...

So although some German verbs with prefixes do separate, this one does not when it means to converse.

Lange here means for a long time.

So the sentence means that the conversation lasts a long time.

Examples:

• Wir unterhalten uns lange. = We talk for a long time.
• Sie haben sich lange unterhalten. = They talked for a long time.

German often uses lange by itself where English uses for a long time.

So:

• lange = for a long time
• den ganzen Abend lang = all evening long

You do not need a separate word equivalent to English for in this sentence.

Yes. Unterhalten here is in the present tense: wir unterhalten.

German present tense often covers both ideas that English separates into:

• we talk
• we are talking

So depending on context, Wir unterhalten uns lange could mean:

• We talk for a long time
• We are talking for a long time
• We have long conversations

English chooses between simple and progressive more often than German does.

Yes, but the meaning is a little broader.

• Wir unterhalten uns lange emphasizes having a conversation / chatting
• Wir sprechen lange just means we speak for a long time and can sound a bit less specifically conversational

If you want the sense of we chat / we have a long conversation, Wir unterhalten uns lange is very natural.

A very common past form is the perfect tense:

• Wir haben uns lange unterhalten. = We talked for a long time.

Notice that in the perfect tense:

• haben is the auxiliary
• uns stays with the verb phrase
• unterhalten goes to the end as the past participle form, which looks the same here

So the sentence structure changes, but the reflexive pronoun is still there.

Yes. If you want to name the other person, mit is very common.

Examples:

• Wir unterhalten uns. = We are talking.
• Wir unterhalten uns mit unseren Freunden. = We are talking with our friends.
• Ich unterhalte mich mit meiner Schwester. = I am talking with my sister.

So mit + dative is the usual way to add the conversation partner.