Inversion

[Belly T]

If I write " There lies a big tree and a moutain", is the sentence grammatically correct?

[Casiopea]

Grammatically, yes.

There lies this and that. <two separate units>
There lie this and that. <one whole unit>

Correction
mountain

[Belly T]

But when to separate, and when to consider it as a whole unit?

[BobK]

But when to separate, and when to consider it as a whole unit?

When Casiopea said 'Grammatically, yes' I think she was suggesting that in that case it would be more likely that the speaker would regard them as separate:

At the end of the trail, there lie a mountain and a tree [separate - two features of the landscape]

At the end of the trail, there lies a tree and a disused well [whole unit - one scene]

But what makes 'a scene', and what bits of it count as 'features', is a very subjective thing. So the speaker decides.

b

[Belly T]

So how about this:
Here come or comes the cars?

[BobK]

Plural subject, so 'here come'.

b