Ramu likes Mangoes.
Here Ramu is the subject, whom we are talking about, likes is the verb since it is the action that is being done here and mangoes is the object, being associated with the action done by the subject. And these three words in the right order makes perfect sense, and hence forms a sentence. If the order was wrong, like- Likes Mangoes Ramu, then it will only be a set of jumbled words and not a sentence, since it does not make any meaning.
Another example would be:
Sonia has a red car.
Sonia here is the subject, has is the verb since it shown the action of owning something and a red car is the object, associated with Sonia and the action that she is doing. Hence, this forms a perfect sentence.