‘A bottle was broken’.

‘A team was established’.

*‘A milk was spilled’.

In the case of mass nouns like ‘milk’, we would normally use ‘some’ as the determiner. However, in speech, ‘a milk’ or ‘a coffee’ commonly refers to ‘a glass of milk’ or ‘a cup of coffee’.

2. Some nouns can be thought of as either countable or mass nouns. For example, you could offer someone ‘a chocolate’ (countable) or ‘some chocolate’ (mass). In the first case, the chocolate is perceived as being one of several in a box, while in the second case, it is probably perceived of as a substance, part of a chocolate bar.

3. Collective nouns like ‘team’, ‘government’ and ‘party’ were traditionally thought of as being singular, and so were used with singular verbs. Nowadays, however, they tend to be considered plural. Therefore, ‘The team were playing’ is now more likely than ‘The team was playing’, the older and more formal usage.