The subject can be seen as the main player in the state-of-affairs: it is the focus of the event, activity or situation described by the predicate, and therefore the subject has a close grammatical relationship with the predicate. This relationship is evident in the way the subject influences the form of the finite verb: a singular subject needs a singular verb form, and a subject in the plural needs a plural verb form. This is called agreement: subject and verb have to agree with each other in person (1st, 2nd, 3rd) and number (singular, plural). Note that languages may have different interpretations of what is singular and what is plural. A good example is the Dutch word politie, which is singular, whereas the English equivalent, police, is plural:
De politie denkt [singular form of the verb] dat de man net in Nederland was aangekomen, omdat hij lichte kleding droeg.
The police think [plural form of the verb]that the man had recently arrived in the Netherlands, because he wore light clothing.
Alan houdt van muziek.
Alan loves music.
Hij speelt gitaar.
He plays the guitar.
De meeste vrienden van Alan houden ook van muziek.
Most of Alan’s friends also love music.
It is sometimes said that the subject of a sentence carries out the action described by the verb, but this is not to be taken literally. The subject of a sentence can perform a variety of roles:
De jongens zijn aan het voetballen.
The boys are playing foorball.
Ik ben verliefd.
I am in love.
Mijn oma krijgt morgen een nieuwe heup.
My grandma is having a hip replacement tomorrow.
Dit jaar duurde de winter heel lang.
This year winter lasted very long.
Feyenoord is door Ajax verslagen.
Feyenoord has been beaten by Ajax.
Only in the first of the above examples does the subject (the boys) carry out an activity. In the second example I undergo an experience and in the third my grandma is a recipient. It is difficult to determine the role of winter in the fourth example, but there is no activity being carried out. The final example is a passive sentence, where the subject undergoes the activity described by the verb. However, what all these subjects have in common is grammatical: they determine the form of the finite verb.