I. Subject-Verb Understanding: The verb in a sentence must be in accordance with its subject.

(i) They both should be either singular or plural.

Eg. - A boy is reading a novel (singular).
The boys are reading a novel (plural)

(ii) In case, the subject is a collective noun, then the verb will take a singular form.

Eg. - The class is making a noise.

NOTE : There are four collective nouns viz. cattle, poultry, police and gentry; with these nouns, we use a plural verb. And there are exceptions to the rule.

(iii) In case, the subjects are connected by 'and'; they require a plural verb.

Eg. - Gold and Silver are precious metals.

(iv) If the subjects are connected by 'or', the verb used will be singular

Eg. - The dog or the pup is sick.

(v) In case there are two different subjects; the verb is put matching the closure subject.

Eg. - Rahul or I am going for a party.

(vi) All the sentences that begin with EACH, EVERYONE and ANYONE will have a singular verb.

Eg. - Every one of the boys loves to ride.
Anyone has a pen, please.

(vii) Confusion between I and Me: Often there is confusion on which form to use when there are two subjects or objects linked with AND, as in these examples:

Eg. - Jenny and me/I joined the chess club.
Jill took Justin and me/I to the shop.

In the first sentence, Jenny and me/I are the subjects of the verb joined. Therefore, the subject pronoun 'I' is considered correct grammatically.

In the second sentence, Justin and me/I are the objects of took. Therefore 'me' is considered correct grammatically.

NOTE: Whenever a comparison is made using than or as, the objective form of Pronoun is used.

(vi) Usage of NEITHER NOR and EITHER OR:

If both the subjects are singular, the verb will also be singular.

Example: Either the mother or the daughter has cooked the meal.

But when one of the subjects, joined by OR or NOR is plural, the verb must be plural and the subject should be placed near the verb.

Example: Neither the teacher nor the students were present.

Similar usage:

While forming a sentence, the structure of the sentence should be kept parallel. If an infinitive is used, then all the phrases should have an infinitive. If a verb is used after it, then we use the objective cases.

Example: She likes to cook, dance and play.

Similar rule is used for a gerund.

Example: She likes cooking, dancing and playing.

II. Repetition Error: Sometimes also referred a 'redundancy', this is the error of writing the same thing twice.

Example:

He returned back from Delhi.
I hardly have any money to give you.

The correct way of saying these should be,

He came back from Delhi.
I have no money to give you.

III. Modifier Error: A common blunder is to leave a participle, without a subject.

Example: Sitting on the gate, a scorpion stung him.

Here, 'sitting' cannot be used for scorpion as it is grammatically incorrect.

So, the correct way of saying should be - Sitting on the gate, he was stung by a scorpion or While he was sitting on the gate, a scorpion stung him.

IV. Comparisons: The comparisons made should be between two similar things, like - The population of London is greater than any other city in India. We are comparing:-

(a) The population of London

(b) Any other city in India.

The correct comparison should be between the populations of both. So, the correct expression should be:-

The population of London is greater than that of any other city in India.

Rule

(a) When comparative degree is used with than, make sure that we exclude the thing compared from the rest of class of things by using the

Eg. He is stronger than any man living. (Incorrect).
He is stronger than any other man living. (Correct).

(b) In superlative degree, we must include the thing compared.

Eg. He is the strongest of all men.