Eloquent means fluent or persuasive in speaking or writing, so “Fluent” is correct.

Harmony means peace and agreement, so its opposite is “Conflict”.

Accommodate is the correct spelling with double ‘c’ and ‘m’.

Has been singing shows Present Perfect Continuous tense.

Do should be replaced with “does” for third person singular.

An “Astronomer” is a scientist who studies celestial bodies.

Burn the midnight oil means working late at night.

Of all indicates a superlative, so “tallest” is correct.

Will be completed is the correct passive structure for simple future tense.

The correct phrase is “married to”.

Generous means kind and giving, so “stingy” is the opposite.

The proverb means it’s better to do something late than not do it at all.

Honest starts with a vowel sound, so “an” is correct.

Present tense in direct speech changes to past in reported speech.

Library is the place where books are kept for reading or borrowing.

Has ate is incorrect; it should be “has eaten”.

Courageous means brave.

Will have finished is the Future Perfect tense.

Kick the bucket is an informal way of saying “to die”.

The correct preposition is “to” when explaining something to someone.

Ancient means very old, so “modern” is its opposite.

Separate is the correct spelling.

The letter was written is past simple passive; active is “She wrote the letter.”

A “Lexicographer” writes or compiles dictionaries.

Neither is singular, so it should be “has”.

Rapid means quick.

The proverb means too many people involved in a job may spoil it.

Must shows obligation.

Transparent means clear; “opaque” is the opposite.

Because shows reason.