は vs が when the subject is modified by a relative clause


Please note that the question is not whether the subject in a relative clause
should be followed by は or が. (In that case I am aware that the thematic は
cannot be used.)

I was reading section II.2, Wa and Ga, of the Structure of the Japanese
Language by Kuno Susumu (1973). The section is a comprehensive discussion of
those two particles and contrasted the use of the particles after the subject
of a sentence/clause. Here is a sentence taken from the section.

> Johnが好きな子 **は** Maryです。

The author intended to illustrate the use of が in the relative clause.
However, what I found interesting was the use of は after 子. I have two
questions.

First, using は to mark the topic of a sentence requires some explicit or
implicit context. In brief, my question is whether the relative clause can
serve as context that enables the sentence "Johnが好きな子 **は** Maryです" be placed
at the beginning of a discourse.

When we say "as for X, ..." we need to make sure that the listener already
knows what X refers to, either from previous context, or from general
knowledge (e.g. X = the sun). The author wrote that the the thematic は
requires the topic to be anaphoric, meaning that it has already been entered
into registry of the present discourse. Obviously we can't begin speech with
"子はMaryです" because the listener wouldn't know what 子 is being talked about.
But when 子 is being modified with the relative clause "Johnが好きな", is that
sufficient to make "Johnが好きな子 **は** Maryです" the first sentence of a discourse?

Secondly, is there a situation that would make

> Johnが好きな子 **が** Maryです。

grammatically correct?

**Edit** : as noted in the comment section, it would sound very abrupt to
begin speech with Johnが好きな子 **は** Maryです. Let's pretend that the social
dimension of communication can be separated from transmission of information.
For the purpose of conveying information only, can a relative clause serve as
context for the subject to be marked with は?




As you suggest, ジョンが好きな is enough for your listener to know you are going to
talk about a specific child as opposed to children in general. However, that's
not the reason は is used instead of が. Children in general can be a topic as
in 子供は無料です whereas 子がメアリーです is no less weird as a standalone sentence than
子はメアリーです.

The question is whether the predicate is matching the topic. In your example,
the predicate メアリーです clearly requires the topic (or topicalized subject) to be
referring to one person. So the listener needs to know which one person you
are referring to and the relative clause does help clarify that. If this
condition is met, 子はメアリーです can also be completely natural.

> 妻の名前はナンシーです。子はメアリーです。

In general, a sentence with a nominal or adjectival predicate tends to sound
neutral with は. This should not be surprising because when you talk about a
characteristic or attribute of something or someone, you would expect your
listener to know what or whom you are talking about.

There are situations where が sounds natural.

> ジョンが好きな子がメアリーです。

This is an answer to who Mary is, rather than who the child John likes (or the
child who likes John, depending on interpretation) is. It’s saying Mary is the
one who John likes (or ditto).

The presence of a relative clause has little to do with which of は and が
sounds more natural.

> 妻の名前はメアリーではありません。子がメアリーです。




As you suggest, ジョンが好きな is enough for your listener to know you are going to
talk about a specific child as opposed to children in general. However, that's
not the reason は is used instead of が. Children in general can be a topic as
in 子供は無料です whereas 子がメアリーです is no less weird as a standalone sentence than
子はメアリーです.

The question is whether the predicate is matching the topic. In your example,
the predicate メアリーです clearly requires the topic (or topicalized subject) to be
referring to one person. So the listener needs to know which one person you
are referring to and the relative clause does help clarify that. If this
condition is met, 子はメアリーです can also be completely natural.

> 妻の名前はナンシーです。子はメアリーです。

In general, a sentence with a nominal or adjectival predicate tends to sound
neutral with は. This should not be surprising because when you talk about a
characteristic or attribute of something or someone, you would expect your
listener to know what or whom you are talking about.

There are situations where が sounds natural.

> ジョンが好きな子がメアリーです。

This is an answer to who Mary is, rather than who the child John likes (or the
child who likes John, depending on interpretation) is. It’s saying Mary is the
one who John likes (or ditto).

The presence of a relative clause has little to do with which of は and が
sounds more natural.

> 妻の名前はメアリーではありません。子がメアリーです。

