A sentence is a block of writing that makes sense on its own. We split writing into sentences using capital letters and full stops, which makes it easier to read.
Look at this 'sentence'.
Peter went home he had his tea.
It is really two separate sentences. The first one tells us that Peter went home and the second one tells us that he had his tea. If we put in a full stop and a capital letter in the right places it becomes much easier to read.
Peter went home. He had his tea.
In this worksheet you can practise putting full stops and capital letters in the right places to show where one sentence ends and the next one begins.