Fill in the blanks with the correct articles: Neil Armstrong was ______ first…

Concept

The definite article "the" identifies a noun as something specific or one of a kind. Two rules apply here: (1) ordinal words such as first, second, last always take "the" because they single out one ranked item; and (2) unique objects that there is only one of — the sun, the moon, the earth, the sky — also take "the". The indefinite articles "a"/"an" do the opposite: they introduce one non-specific member of a group, so they cannot sit before an ordinal or a one-of-a-kind noun.

Application

• First blank — the word "first" is an ordinal, so by rule (1) it must take "the": "the first man".

• Second blank — "moon" is a unique celestial body, so by rule (2) it must take "the": "the moon".

Both blanks therefore take "the", giving "the first man to walk on the moon".

Contrast

• "a, the": "a" before the ordinal "first" is wrong — ordinals are never indefinite.

• "an, the": "an" before "first" is wrong twice over — "first" starts with a consonant sound and is an ordinal needing "the".

• "an, a": indefinite articles in both blanks ignore that "first" is ranked and "moon" is unique.