Subjects are noun phrases, and usually have more than one word in them, but they can be just one word, if there are no modifiers. Subject is a grammatical concept restricted to languages with nominative-accusative systems,
like most Indo-European languages. Languages like Basque, Georgian, Quiché, or Pitjantjatjara,
which have absolutive-ergative systems, do not support a meaningful concept of Subject.
Virtually all tensed English clauses (including all simple sentences), require a Subject constituent.
Besides its position before the verb phrase, the grammatical properties of a Subject include:
- number agreement with the verb phrase
The ladies are arriving ~ The lady is arriving
- inversion with auxiliary in questions
The old man in the trenchcoat is coming => Is the old man in the trenchcoat coming?
- pronominalization in tag questions
Those guys are the ones, aren't they?
- upstairs control of A-Equi deletion, plus downstairs deletion by Equi
Bill wanted to see the painting = Bill wanted (for Bill) to see the painting.
- promotion by Subject-Raising (often obligatory)
*For there to be beer in the fridge tends => There tends to be beer in the fridge.
- movement and optional deletion by Passive
Acme Construction erected this building in 1936 => This building was erected in 1936.
- contraction with auxiliary (especially pronouns)
The old woman has/is gone now => The old woman's gone now
In addition, there are semantic criteria governed by predicates. Most predicates will only accept certain types of noun phrase as subjects, and lots of tests can be fashioned with different verbs.