The easiest way to know which to use is to determine, is the new or significant information before or after the particle?

Let’s take as an example: “I like strawberries”. Do you use 苺が好きや or 苺は好きや? Let’s ignore first the misconception that 好き always uses が (it’s not true!). Which we use depends on the context, ie what comes before.

Let’s take the scenario:

You’re in the fruit aisle in a supermarket with your friend. She asks: 何が好き? What do you like?

In this case we reply with: 苺が好きや. I like strawberries.

The new information is before the particle, ie we are already discussing what you like, so that’s not new. The new information is “strawberries”.

So if the new or most important information comes before the particle, the particle you need is が.

What if your friend says instead: 苺あるわ。They have strawberries.

Your response would be: 苺は好きや! I like strawberries!

And this time we use は, because we have already brought up the topic of strawberries. The new information is that you like them, and that comes after the particle. So in this case, you use は.

To summarise:

• New information before the particle → use が
• New information after the particle → use は

The same applies to deciding whether to use を or は:

• New information before the particle → use を
• New information after the particle → use は