The correct sentence should be:

It was he who encouraged me when I was feeling low.

Explanation: In formal English, the pronoun "he" is used after the verb "was" to function as the subject complement in this sentence. Additionally, the objective form of the pronoun "me" should be used, and the verb "encouraged" should be followed by the object pronoun "me."

The correct sentence should be:

She was too startled at first to say anything, but finally, she blurted out the truth.

Explanation: The phrase "to say something" is the appropriate choice here. The verb "startled" is typically followed by an infinitive verb form, so "to say" is used. Additionally, the sentence benefits from proper punctuation, including a comma after "first" and another after "finally."

The correct sentence should be:

On winter evenings, a haze of smog hangs in Delhi.

Explanation: The verb "hangs" agrees with the singular subject "haze." In the original sentence, "hang" is incorrect in this context.

The correct sentence should be:

There were ten children in the class, six of whom were crying.

Explanation: In the original sentence, "which" is incorrect because it refers to things, not people. Instead, "whom" should be used to refer to the children, and the verb "was" should be corrected to "were" to agree with the plural subject "six children."

Explanation:

• "You need": This part of the sentence indicates a requirement or necessity for the person being addressed.
• "to take": This phrase is followed by the base form of the verb, indicating the action that needs to be performed.
• "some guidance": This part specifies what is needed - guidance in the subject of Chemistry.
• "in Chemistry": This phrase clarifies the specific area where guidance is required.

The sentence is concise and effectively communicates the idea that the person being addressed should seek guidance in the subject of Chemistry. There is no need for any substitution or improvement in this sentence.

The correct sentence should be:

Monika has been learning French for a long time.

Explanation: The verb form "learn" should be changed to "learning," and the auxiliary verb "has been" should be used to indicate the ongoing action of learning that has been happening for a period of time.

The correct sentence should be:

If somebody attacks you, you need to be able to defend yourself.

Explanation: The verb "needs" should be changed to "need," and the word "be" should be included for the sentence to be grammatically correct. The corrected sentence indicates the necessity of being able to defend oneself if attacked.

The correct sentence should be:

The doctor bade me take the medicine immediately after breakfast.

Explanation: In this sentence, the base form of the verb "take" should be used after the verb "bade." The construction "bade me taking" is incorrect; it should be "bade me take" to convey the intended meaning.