This might seem very straightforward, but it’s worth having a clear definition in your mind. Use the following definition, taken from the Cambridge English Dictionary: ‘A sentence is a group of words, usually containing a verb, that expresses a thought in the form of a statement, question, instruction, or exclamation, and starts with a capital letter when written.’ It’s worth noting that there is ‘usually’ a verb – it’s rather difficult to think of sentences without verbs until one realises that an exclamatory sentence needs no verb. An example:
What a disaster!