Grammar Guide: Difference between が and は preceded by a subject

As a native Japanese speaker, I often find many Japanese learners being so much confused with the difference between particle が and は. Some learners highly recommend Tae Kim’s Japanese Grammar Website. But his article about が and は isn’t so good, unfortunately. (Note: I’m not denying all of his articles because I haven’t read through them all yet.)

From different points of view, I explained about the difference on several pages including on external websites. Let me compile them here as an alternative guide for you.

が is for a vertical comparison whereas は is for a horizontal comparison when they are preceded by a subject.
→ Read this discussion first.

Example sentences with explanation:

  1. What’s the difference between この鍋鉄でできています and この鍋鉄でできています (both means “this pot is made from iron”)? answered on HiNative
  2. What’s the difference between 私毎日、部屋の掃除をします and 私毎日、部屋の掃除をします (both means “I clean up the room(s) everyday”)? answered on the same page of the first case.
  3. What’s the difference between 我が社では車を作ります and 我が社車を作ります (both means "our company makes cars)? answered on HiNative – Hint: it’s a looooong series of threads. Jump to my answer posted on Feb 9, 2021.