Place of adjectives in a sentence

Question

This topic confuses me very much. I'm aware that the meaning of words in a sentence depends on its place, but I can't still completely grasp the idea/topic. Could you help me with the next sentences, please?

(a) Look out! There's a car coming!
(b) I came across a group of children playing.

The both look to me as incomplete, as if there's an implicit, deleted part of a sentence. Is this right?
As if they should be kind of:

(a)
There's a car coming along the street. /
There's a car coming toward you.
(b)
I came across a group of children playing in the playground. /
I came across a group of children playing next to my house.

-shouldn't they?

I have no questions on these full sentences, but their shorter form confuses me.

The second question is whether the words "coming" and "playing" can stand before the noun?
Kind of:

(a) Look out! There's a coming car.
(b) I came across a group of playing children.

Are these sentences correct? If they are, do they mean the same as ones above and sound native?

And the last question:
We know a phrase about English "the shorter the better".
So why "There's a car coming!" rather than "A car is coming"?
Are the both correct, do they mean the same and sound native?

(a.1) Look out! There's a car coming!
(a.2) Look out! A car is coming!

Please, could you help me with all these questions?

Answer 1

There is nothing incomplete about the first group of sentences you posted; they are completely normal. At most, you could say that there is an implied "which is" or "who is", like

I came across a group of children who were playing.

or maybe

I came across a group of children who were in the act of playing.

But the meaning is simply

I came across a group of children, and they were playing.

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For your second question, yes, both of those are grammatically correct. In these sentences, "playing" or "coming" is something called a participle, which is essentially a verb form used like an adjective. The children are playing, so they are playing children. "Coming car" is unusual - we would usually say oncoming car - but it isn't grammatically incorrect.

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For your third question, yes, shorter is usually better when it comes to spoken English! But both of your answers are correct and can be found in live usage. For example,

"There's A New World Coming" (http://www.goodreads.com/book/show/578780.There_s_a_New_World_Coming)

and

"A Blizzard Is Coming" (http://www.wnyc.org/story/blizzard-coming/)

They differ at most only very slightly in meaning: "There's a X coming" means that there exists an X, and its action or state of being is "coming". "An X is coming" means that a specific example of an X has a state of "coming". In practice, they're pretty close to interchangeable. I would regard as completely normal both

Hearing the sound of footsteps on the driveway, he turned to look. "There's someone coming," he said.

and

Hearing the sound of footsteps on the driveway, he turned to look. "Someone's coming," he said.

Answer 2

"Coming" there means "moving towards us". So there's no need for a complement such as "along the street".

In American English, Look out! A car's coming! is a normal warning to someone who might be about to step off the curb into the street without looking.

We don't have to say "without looking down the street" either, because "without looking" means "without looking in the direction(s) one needs to look in".

Can you juggle? Can you juggle without looking? (this example brings in gerunds, which are like verb-nouns or noun-verbs, but the underlying principle is much the same.)

In the fable, Chicken Little does not say "The sky is falling down to the earth!" but simply The sky is falling!

Answer 3

There's nothing particularly incomplete about these sentences. Sure, you could add additional information. The word "coming" implies "towards us" or "towards you" or "towards me". You can't say, "There's a car coming away from us", you'd have to say "... going away from us." So "There's a car coming TOWARDS YOU" is a little redundant. You might add the "towards you" for emphasis, but it's not necessary. Similarly, cars normally travel along a street, so it is rarely necessary to say that. If the car was NOT coming along the street, you might want to specify. Like, "Look out! There's a car coming out of that driveway!" or "There's a car coming across the grass!" But again, it's not GRAMMATICALLY necessary.

Similarly with "childen playing". Sure, you COULD give additional detail. You could say where they are playing or what they are playing. But it's not necessary to make the sentence grammatically correct.