The subject must agree with the verb in number and person. Basic rules of sentence agreement are given here.
Singular subjects take is (present tense) and was (past tense). Plural subjects take are (present tense) and were (past tense). Singular verbs take the marker –s with them. Plural verbs do not take –s.
I am going to the library. (Here the verb am agrees with the subject I.)
He is going to the library. (Here the verb is agrees with the third person singular subject he.)
They are going to the library. (Here the verb are agrees with the third person plural subject they.)
You are a good boy. (Here the verb are agrees with the second person singular subject you.)
It is very expensive.
Alice was reading.
The children were playing.
In the present tense, we use am with I, is with he/she/it and are with they/we/you. In the past tense we use was with I/he/she/it and were with we/they/you.
Sometimes the subject may consist of two or more nouns, pronouns, or noun phrases. The verb has to be singular when the nouns together express one idea.
Bread and butter is what they want.
Slow and steady wins the race.
When two or more nouns refer to the same person or thing, we use a singular verb.
A black and white cow was grazing in the field.
The presence of phrases such as along with and as well as do not affect the rules of sentence agreement.
John as well as you is correct. (Here the verb agrees with the subject John.)
He as well as I is coming. (Here the verb is agrees with the subject he.)
John along with Peter has arrived. (Here the verb has agrees with the subject John.)
A verb which is limited by the number and person of its subject is called a finite verb. A verb which is not limited by the number and person of its subject is called a non-finite verb.
All verbs in the indicative, imperative and subjunctive moods have to be finite. Notice how the verb changes its form in accordance with the number and person of the subject.
They fly kites.
He flies kites.
I fly kites.
You fly kites.
She flies kites.
Participles, infinitives and gerunds are called non-finite verbs because their form does not change in accordance with the number and person of the subject.
I like swimming.
She likes swimming.
He likes swimming.
They like swimming.
You like swimming.
From the above examples you will have noticed that the present participle swimming does not change its form in accordance with the change in the number and person of the subject.
I don’t want to wait.
He doesn’t want to wait.
She doesn’t want to wait.
They don’t want to wait.
You don’t want to wait.
In the examples given above, the verb do changes its form according to the number and person of its subject. It is, therefore, a finite verb. The infinitive to wait doesn’t change its form according to the number and person of the subject. The infinitive to wait is therefore a non-finite verb.