Explanation

Honesty never goes to waste.

The subject "Honesty" is singular and uncountable,

And the sentence is in the present simple tense, expressing a universal truth or general fact.

In present simple tense: Singular subject → verb + s/es

Explanation

His father is writing a book. (Present continuous tense)

 If the action is a regular or habitual activity

 Is writing – if it’s happening right now 

Explanation

 When I met him, he was going to college.

The sentence is referring to a past event (When I met him),

So the action happening at that moment in the past should be in the past continuous tense: He was going to college.

Explanation

I have been helping him for many years.

میں کئی سالوں سے اس کی مدد کر رہا ہوں۔

The sentence uses "for many years", which indicates an action that started in the past and is still continuing.

For such cases, we use the present perfect continuous tense:

$Subject+have/hasbeen+verb-ing$

Explanation

I hope they will not have left the city before I reach there.

مجھے امید ہے کہ میرے وہاں پہنچنے سے پہلے وہ شہر چھوڑ کر نہیں گئے ہوں گے۔

The sentence talks about a future action ("before I reach there") 

Future completed action is the future perfect:

will not have + past participle

"Will not have left" shows:

A completed action in the future

That you hope they are still there when you arrive