Usually no article before plural nouns.
I don't like dogs.
I don’t like dogs means میں کتوں کو پسند نہیں کرتا ہوں
'An' would be the right article.
"The Atlantic" is a specific ocean, so the definite article "the" is used.
We use "the" before names of oceans, seas, and rivers.
We need to protect the environment/earth.
ہمیں ماحول/زمین کی حفاظت کی ضرورت ہے۔
Usually Article the is used before unique things like Sun, Earth, Moon etc.
“Best” is the superlative degree of Adjective.
Article “the” is generally used before a superlative degree.
He is the best soldier.
"A good dentist" is used for the first general mention (indefinite article).
"The dentist" refers to the same specific person already introduced (definite article).
“European” starts with a vowel letter but a /juː/ sound, so “a” is correct
A mile is a unit of measurement for distance.
A litre is a unit of measurement for liquid — this is a similar relationship.
The correct sentence is: He was generous to the helpless orphans.
"The" is used to specify a particular group (helpless orphans in this case).