What is the difference between these sentences used が and は


I'm trying to translate an English sentence into Japanese , but i got some
problems and confuse which should i use between が or は . So please tell me
which is the correct one .

  1. ジョンさんは休日にいつも東京にいる。
  2. ジョンさんが休日にいつも東京にいる。

Also 1. 彼は傷口にそっと触れる。 2. 彼が傷口にそっと触れる。




I think one way to see the difference would be...

  1. ジョンさん **は** 休日にいつも東京にいる。  
_lit._ As for John / Speaking of John, he is usually in Tokyo on holidays.
(and someone else among the people we're talking about may also be in Tokyo.)

  2. ジョンさん **が** 休日にいつも東京にいる。  
(Of all the people we are talking about) It is John (not someone else) who is
usually in Tokyo on holidays. / John is the one who is usually in Tokyo on
holidays.

The は is the **topical/[thematic
は](https://en.wikipedia.org/wiki/Japanese_grammar#Thematic_wa)**. Sentence 1
is usually said when ジョンさん is the topic of the conversation, i.e. ジョンさん has
already been mentioned before this sentence is uttered, or the interlocuter(s)
is/are expecting ジョンさん to be probably mentioned.

The が is the **[exhaustive
が](https://en.wikipedia.org/wiki/Japanese_grammar#Exhaustive_ga)**. Unlike wa,
the subject particle ga nominates its referent as the sole satisfier of the
predicate ([quoted from
here](https://en.wikipedia.org/wiki/Japanese_grammar#Exhaustive_ga)). So you'd
usually say like ジョンさん **が** (東京に)いる (It is John who is in Tokyo) as a reply
to a question 誰が東京にいますか？

* * *

  1. 彼 **は** 傷口にそっと触れる。 
  2. 彼 **が** 傷口にそっと触れる。 

If this line is from a story/novel, then the present form 触れる is the
**historical present** (史的現在). Both sentences are grammatically correct, and
one would be preferred over the other depending on the context.  
You'd tend to use 彼は when 彼 has just done some other things or has just been
mentioned, whereas you'd tend to use 彼が when you want to specifically say that
it's 彼, not someone else, who does that action. (Thanks to @Sjiveru)  
が also tends to be used in relative clauses and subordinate clauses, e.g. 「彼
**が** 傷口にそっと触れると、彼女は～～」「彼 **が** 傷口に触れた瞬間」  
The は can also be the **[contrastive
は](https://en.wikipedia.org/wiki/Japanese_grammar#Exhaustive_ga)**. You can
use the は to contrast 彼 (or 彼's action) with someone else (or their action).
E.g. 「 **彼女は** 治療を拒んだが、 **彼は** (not 彼が)傷口にそっと触れて…」

