Explanation

"A" is used before a singular, countable noun when mentioned for the first time.

"A letter" fits because it refers to one unspecific protest letter.

Explanation

"Either" refers to one of two options, so it takes a singular verb.

Therefore, "is" agrees correctly with the singular subject "Either answer."

Explanation

"As well as" introduces extra information that modifies the main subject.

The verb agrees with the main subject only, which is singular ("The player").

Therefore, the singular verb "wants" is correct.

Explanation

"The environments" refers to specific natural settings already known or implied.

Use of "the" is proper when talking about general threats to particular environments.

Explanation

This sentence belongs to Present Indefinite Tense. 

Active voice:  This footpath is used all the time

Passive voice: People use this footpath all the time.

Note: Do not need use subject in passive voice when it is understood in the sentence. 

Explanation

Could is a modal verb.

Could can be used for polite/formal request.

Could I have some more tea please?

کیا میں مزید چائے پی سکتا ہوں؟

Explanation

Here three years indicate that this sentence belongs to Present Continuous Tense. 

We use has been + ing form of verb with singular subject in this tense.

She has been nursing her youngest child for three years

Explanation

Formula of First conditional sentence: If/when + present simple >> will + infinitive

If you work in collaboration, you will make rapid progress.

اگر آپ تعاون سے کام کریں گے تو آپ تیزی سے ترقی کریں گے

Explanation

Here was cheating shows past tense.

We use second form of verb (knew) in simple sentence of Past Indefinite Tense.

The examiner knew that the candidate was cheating.

Explanation

"Worth seeing" correctly describes something valuable or interesting to look at.

The structure "a site worth seeing" is grammatically correct and commonly used.