When do i use は (わ) and when do i use が?

See a translation

●[は and が]
･https://youtu.be/FknmUij6ZIk
･https://www.thoughtco.com/japanese-particles-wa... (Explanation)
･http://www.guidetojapanese.org/learn/grammar/pa... (Explanation)
･http://www.guidetojapanese.org/blog/2005/02/05/... (Explanation)
･http://www.learn-japanese-adventure.com/japanes... (Explanation)
･http://kwhazit.ucoz.net/ranma/g_part.html#wa1 (Explanation)
･http://www.japanese-language.aiyori.org/article... (some examples)
･http://nihonshock.com/2010/02/particles-the-dif... (Our image/view)
･https://8020japanese.com/wa-vs-ga/ ("The true purpose of [は]")

日本語での説明↓
･https://www.alc.co.jp/jpn/article/faq/03/19.html
･http://learn-the-basics-of-japanese.blogspot.jp...
･http://learn-the-basics-of-japanese.blogspot.jp...
･http://learn-the-basics-of-japanese.blogspot.jp...


There are 主題(theme, topic) and 主語(subject).
1. John ate that apple. 
↑ ↑
topic object
subject

1a. ジョンはあのリンゴを食べた。
↑ ↑
topic object
subject
Speaking about John, (he) ate that apple.
As for John, (he) ate that apple.

1b. ジョンがあのリンゴを食べた。
↑ ↑
subject object
Topic(/Theme) is not written here.

2. あのリンゴはジョンが食べた。
That apple, John ate (it).
↑ ↑
topic subject
object
Speaking about that apple, John ate (it).
As for that apple, John ate (it).



Q1: What did John do?
A1: ジョンはあのリンゴを食べた。
(Speaking about / As for) John, (he) ate that apple.

Q2: Who ate that apple?
A2: ジョンがあのリンゴを食べた。
John ate that apple. (/ John did.)

Q3: (I put my apple on this table. But, it is not there.) Where is that apple? (Do you know that?)
A3: あのリンゴはジョンが食べた。
(Speaking about / As for) that apple, John ate (it).


e.g. 1a
私はラーメン。
↓[without context]
I'm ramen. (strange sentence)

e.g. 1b
A: 俺は寿司を食べたい。あなたは？
I want to eat sushi. What about you? [context]
B: 私はラーメン。(natural sentence)
(Speaking about / As for) me, (what I want to eat is) ramen.

e.g. 2
こんにゃくは太らない。(natural sentence)
It doesn't mean "Konjak doesn't get fat."
[こんにゃく(Konjak) is a food which has very low calorie.]
↓ implying
こんにゃくは、いくら食べても太らない。
(Speaking about / As for) konjak, no matter how much (we) eat (it), (we) don't get fat.

Picture ｢AはB。｣