Why can't pronouns be a subject?

Question

I wish to ask about this comment posted by Janka.

You can deduce that in this case because a pronoun cannot be a description of the subject.

Could someone explain why this is ? And then perhaps some important results due to this?

Answer 1

You are mixing up these two different things:

• Subject
• Description of a subject

Sentence 1 is the sentence that Janka wanted to explain with his statement. Sentence 2 is another sentence that uses the same pattern:

1. Er ist ein sehr lustiger Clown.
2. Ich bin Österreicher.

This is the pattern:

<subject> <copula> <description of the subject>.

A copula is a certain kind of verb that does not indicate some action (like verbs usually do) but some kind of identity. Copulas are »sein«, »bleiben«, »werden« (to be, to stay, to become) and some others like »heißen« (to be named). And the <description of the subject> in sentences with a copula is always in nominative case, i.e. in the same case as the subject itself, which was the topic of the linked question. But the <description of the subject> is not an object (although at first glance it looks like one) but a predicative expression.

In the sentences 1 and 2 the personal pronouns »er« and »ich« are the subjects. Having a personal pronoun as a subject is very common. You find pronouns as subjects also in sentences that don't use copulas:

Er schläft. (personal pronoun)
Das stimmt. (demonstrative pronoun)
Niemand hat den Tee getrunken. (indefinite pronoun)
Wer ist mit dem Bus gekommen? (interrogative pronoun)
Meinen habe ich gestern verkauft. (possessive pronoun)

Therefore, the assumption on which the question "Why can't pronouns be a subject?" is based is wrong. The question is therefore as meaningless as "Why is the moon made of cheese?".

But Janka still was wrong. He wrote, »a pronoun cannot be a description of the subject«. And this is not true:

Mein lieber Schwager Walter White, ich habe jetzt endlich herausgefunden, wer dieser Heisenberg ist. Das bist du! Du bist Heisenberg!

The last two sentences (»Das bist du! Du bist Heisenberg!«) both match perfectly the pattern we had here in the sentences 1 and 2. The subject of both sentences are pronouns (»das« is a demonstrative pronoun, »du« is a personal pronoun).

But in the bold marked sentence also the <description of the subject> is a pronoun (»du« is a personal pronoun).

In many sentences who's verb is a copula it is ambiguous which of the two parts in nominative case is the subject and which is the description of the subject (like in »Aprikosen sind Marillen«). You can argue that this is the case in »das bist du« too, but it doesn't matter. Everything in this sentence that is not a verb is a pronoun. So we learn from this sentence:

• Pronouns can be subjects.
• Pronouns can be descriptions of subjects.

Answer 2

My comment was about personal pronouns specifically. I should have written it that way.

Personal pronouns can be subjects, but they can't be descriptions of subjects for the simple reason that personal pronouns do not describe anything. They are mere pointers to a person. They don't carry meaning.