Articles

'The United States' is a collective proper noun.

Name of a continent (no article).

Musical instrument usage.

Superlative degree.

Unique planet.

Standard measurement idiom.

'Expert' starts with a vowel sound.

Name of a disease in general.

Certain diseases like 'the flu'/'the measles' take 'the'.

Correct vowel/consonant sound sequence.

Single mountain peak.

'Honor' has a silent 'h', starting with a vowel sound.

'European' starts with consonant sound /y/.

'Umbrella' starts with a vowel sound.

'Useful' starts with consonant sound /y/.

General subject usage (no article).

Specific history requires 'the'.

Standard idiomatic usage 'have a headache'.

Unique heavenly body.