wǒ gēn péngyou tán tiānqì.

Here 跟 most naturally means with. It introduces the person you are talking with:

我 跟 朋友 谈 天气
= I talk about the weather with a friend / with friends

Chinese 跟 can also mean and, so the sentence is a little ambiguous by itself, but in this pattern it is very commonly understood as with.

A very common Chinese pattern is:

Subject + 跟/和 + person + verb + topic/object

So:

• 我 = subject
• 跟朋友 = with a friend / with friends
• 谈 = discuss, talk about
• 天气 = weather

English often puts with later, but Chinese usually puts that phrase before the verb.

Because 谈 can take the topic directly.

So Chinese can say:

• 谈天气 = talk about the weather
• literally, something like discuss weather

This is normal in Mandarin. You do not need a separate word matching English about here.

By itself, 朋友 does not tell you clearly whether it is singular or plural. It can mean:

• a friend
• friends

Context decides.

If you want to be specific, you can say:

• 一个朋友 = one friend
• 几个朋友 = several friends

Mandarin does not have articles like English a, an, and the.

So 朋友 can mean a friend, the friend, or friends, depending on context, and 天气 can mean the weather without needing a separate word for the.

This is very normal in Chinese.

Yes. 我和朋友谈天气 is also natural.

Both 跟 and 和 can mean with in this kind of sentence. Very roughly:

• 跟 often feels a bit more conversational
• 和 can feel a bit more neutral

But both are common and correct.

These verbs are related, but not identical:

• 说 = say, speak
• 谈 = discuss, talk about
• 聊 = chat

In this sentence, 谈 fits because there is a clear topic: 天气.

Some comparisons:

• 说天气 sounds less natural here
• 谈天气 = discuss the weather
• 聊天气 = chat about the weather, more casual

So 谈 is correct, though 聊 would also be possible in a more casual tone.

Chinese verbs do not change form for tense the way English verbs do.

So 谈 by itself does not tell you past, present, or future. You usually learn that from context, time words, or particles.

For example:

• 我跟朋友谈天气。 = I talk / am talking / talk about the weather with a friend, depending on context
• 昨天我跟朋友谈天气了。 = Yesterday I talked about the weather with a friend
• 明天我跟朋友谈天气。 = Tomorrow I will talk about the weather with a friend

Measure words are needed when you count or specify nouns, for example:

• 一个朋友 = one friend

But here 朋友 is not being counted. It is just part of the phrase 跟朋友 = with a friend / with friends.

So no measure word is needed.

Because the second syllable, you, is usually pronounced with a neutral tone in 朋友.

So the usual pronunciation is:

• péngyou

not

• péngyǒu

This is very common in everyday Mandarin. Neutral-tone syllables are lighter and shorter.

A careful pinyin reading is:

wǒ gēn péngyou tán tiānqì

A few helpful points:

• 我 (wǒ) is third tone
• 跟 (gēn) is first tone
• 朋 (péng) is second tone
• 友 (you) is neutral tone
• 谈 (tán) is second tone
• 天 (tiān) is first tone
• 气 (qì) is fourth tone

In natural speech, wǒ often sounds like a low third tone before the next syllable, rather than a full dipping-rising third tone.