が is considered to be the subject marker in Japanese. This means that it marks someone or something that is performing an action, or it identifies someone or something as having a specific feature (this is usually a feature that the listener is not aware of).

• 父（ちち）が働（はたら）く。
  My father works.
• 父（ちち）は背（せ）が高（たか）い。
  My father is tall.

In both of these sentences, が puts focus on the person as being ‘the one’ who is performing an action/existing in some way (as opposed to any other person). Because of this, が is usually said to highlight what comes before it, while は highlights what comes after it.

• コーヒーが好（す）きです。
  I like coffee. (I specifically like coffee)
• テストは嫌（きら）いです。
  I dislike tests. (In general, I don’t like tests)

が always shows the ‘target’ of a statement, when other targets also exist. This is why が is used to describe situations that are happening ‘in the moment’.

• 近所（きんじょ）の子供（こども）たちが走（はし）る。
  The kids in the neighborhood run. (Not all the time, but sometimes)

Because the event in this sentence is only one short moment out of many possible ‘points in time’ that could be described, が will be used.