Learn Spanish (Spain): El abuelo va a comprar un casco. - The grandfather is going to buy a helmet.

Hay que means 'one must' or 'it is necessary to' and is always impersonal, meaning it doesn't specify exactly who has to do the action. Since this sentence tells us exactly who is doing the action (el abuelo) and he is just going to do it rather than having a general obligation to do it, we use va a instead.

What verb form is va a comprar?

This is the 'near future' tense, built using the verb ir (to go) + a + an infinitive verb. Here, va is 'he goes', a is 'to', and comprar is the infinitive 'to buy'. Together, it translates perfectly to the English 'is going to buy'.

Why does it say el abuelo instead of just abuelo? In English we often just say 'Grandpa is going to buy a helmet.'

In Spanish, when you are talking about a family member rather than talking directly to them, you usually need the definite article (el or la). So 'Grandpa' becomes el abuelo. If you were talking to him directly, you would drop the article.

Does casco only mean helmet?

While casco is the standard word for a safety helmet for bikes or motorcycles, it has a few other common meanings. For example, casco antiguo means the historical center or 'old town' of a Spanish city. In this sentence, the context makes it clear he is buying a helmet.