Full stops are used at the end of a sentence. Here are some examples:

1. The dog chased the ball. [simple sentence]

2. The dog chased the ball across the park. [simple sentence] 
3. Quickly, with its tongue hanging out, the dog chased the ball across the park and got tangled in the legs of a woman pushing a pram. [complex sentence]

Ooh! Fun Game
1. Write a simple sentence and punctuate it.
2. Add ‘bits’* onto the beginning and end to make it as long as you can. Separate out the ‘bits’ with commas using the example above to help you. The best, longest sentence wins.
*bits = ‘phrases’ and ‘clauses’.

Back to the Boring Stuff

‘-the dog chased the ball-‘ is a main clause. This means it can stand alone as a proper sentence, as in example 1. above. Any sentence must have a main clause in it. It may also have other bits added on (as above).
How do I bake up a main clause?
A main clause must have a subject* and a verb. It may also have an object.
e.g. I slept. (subject + verb)
I slept on the floor. (subject + verb + object)
1. The verb tells us ‘what happens’.
2. The subject – which is a noun or pronoun – is the thing (or person) that does the verb.
3. The object – which is also a noun or pronoun – is what (or who) the verb happens to.
*unless it’s a command, e.g. ‘Go away!’ or ‘Write a simple sentence and punctuate it.’

Is There Anything Else I Need to Know?
Sentence order in English is always Subject – Verb – Object* aka SVO. This tells us instantly who did what to whom, and is hugely important. If you don’t believe me, look at the example below: *(if an Object is present)

e.g. The boy ate the apple.

The apple ate the boy.

The meaning of each sentence is very different even though all the words are exactly the same. Interestingly, pronouns change form if they’re subject or object. You can’t switch them in the same way:
I spoke to him. >
Him spoke to I. (Eek!)

He spoke to me.  (Aah!)

I = subject        me = object
He = subject   him = object

Interesting Fact
Though it sounds odd to native English speakers, SOV is actually a more common word order worldwide, e.g. The dog across the park ran

Why does a sentence need a subject?
We need to name the subject because we need to know whodunnit – like when someone eats all the biscuits. We want to find out who (Dad) or what (the dog) ate them.

Why does a sentence need a verb?
If the culprit ‘eats‘ the biscuits, it’s a big difference from if they ‘smash‘, ‘crush‘, ‘catapult‘ or choke on‘ them. A sentence needs a verb because if nothing happens, why bother speaking (or writing) about it?

Why do some sentences need objects?
If someone eats all the biscuits, that’s bad. We’d be more upset to discover they ate the birthday cake we’ve just baked for tomorrow. If they ate poison, we need to call an ambulance.

One Common Punctuation Blunder to Look Out For –
Mashing Sentences Together:
The dog chased the ball across the park the ball rolled under a bush and he couldn’t get it out.

Aargh! Why do kids do this?
They’re following a train of thought about a dog and a ball. What they haven’t realised is that there are actually two sentences here: one about the dog chasing the ball, and another about the ball rolling under the bush.
The dog chased the ball across the park. The ball rolled under a bush and he couldn’t get it out.

Urgh! But there are two objects in the first sentence.
That’s fine. The dog can chase the ball, a man and his own tail, all in the same sentence, if he likes. 
e.g. The dog chased the ball, a man and his own tail.

But there are two subjects in the second sentence!
You can link as many subjects as you want (within reason) with words like: and, but, and because.

You could even correct the example above by doing this:
The dog chased the ball across the park and the ball rolled under a bush and he couldn’t get it out.
It’s not very elegant though!

The problem with the original example is highlighted here:
‘The dog chased the ball across  the park the ball rolled under a bush and he couldn’t get it out.’

It’s ‘the park the ball’ that makes me cringe. Now you need to teach your kids to cringe when they see it too – preferably not by punishment beatings.

They need to look for two nouns (or pronouns) jammed horribly close together and followed by a verb.
I hit the  man he hit me back.
I ate a  cake it was very nice.

Just underline every place they do it, and make them write it out again. They’ll soon snap to attention.

Please note: I don’t always write in proper sentences, though I’m well aware of the rules. Feel free to send me a comment on my own punctuation blunders.