Explanation

Don’t take his argument to heart.

The phrase "take something to heart" is an idiom, meaning to take something seriously or be deeply affected by it emotionally.

Uses "to" because it describes the direction of the action—

Explanation

The correct phrase is keen on when expressing interest or enthusiasm.

“She is keen on learning new languages” is the grammatically correct construction.

Explanation

Third conditional uses had + past participle in the if-clause (If I had left earlier…).

It describes an unreal past situation with a missed result (…I would have caught the train).

Explanation

"Lucid" means expressed clearly; easy to understand.

Since "everyone understood immediately," the explanation must have been clear and easy to follow. 

Explanation

He devotes much of his time to planning for the future 

 وہ اپنا زیادہ تر وقت مستقبل کی منصوبہ بندی میں صرف کرتا ہے

Subject: "He"

Verb: "devotes"

The object of the verb: "Much of his time"

Gerund Phrase (acting as a noun): "Planning for the future"