Famous religious places like the Great Mosque of Cordoba always take 'the'. The definite article emphasizes their uniqueness and identity. 'A Great Mosque' or 'an Great Mosque' would be incorrect. 'No article' would also make the sentence wrong. Hence, 'the Great Mosque of Cordoba' is correct.

The word 'egg' begins with a vowel sound, so 'an' is correct. This is the first mention of the egg, making the indefinite article suitable. 'A egg' would be grammatically wrong. 'The egg' would mean one already known. Thus, 'an egg' is the right choice.

Religious texts like the Quran are unique and always take the definite article 'the'. This shows respect and accuracy in usage. 'A Quran' or 'an Quran' would be ungrammatical. 'No article' would also be incomplete. Therefore, 'the Holy Quran' is correct.

Although 'useful' begins with a vowel letter, it starts with the consonant sound 'yoo'. This makes 'a' the correct article. 'An useful' would be grammatically incorrect. Since it is the first mention, the indefinite article fits. Therefore, 'a useful idea' is correct.

Names of rivers always take the definite article 'the'. The River Nile is a specific geographical name. 'A River Nile' or 'an River Nile' would be ungrammatical. 'No article' would also make the sentence incorrect. Hence, 'the River Nile' is correct.

The word 'hour' begins with a silent 'h' and therefore starts with a vowel sound. This requires the use of 'an'. 'A hour' would be grammatically incorrect. 'The hour' would mean a specific one already mentioned. Thus, 'an hour-long journey' is correct.

Historical events always take the definite article 'the'. The Battle of Badr is a specific event, so it must be introduced with 'the'. 'A' or 'an' would be ungrammatical. 'No article' would also make the phrase incomplete. Therefore, 'the Battle of Badr' is correct.

The noun 'owl' begins with a vowel sound, so 'an' is required. This is the first mention, making the indefinite article correct. 'A owl' would be grammatically incorrect. 'The owl' would imply one already known. Hence, 'an owl' is correct.

The noun 'loaf' begins with a consonant sound, so 'a' is correct. Since the loaf is being mentioned for the first time, the indefinite article is appropriate. 'An' would be wrong due to pronunciation. 'The' would imply a specific loaf already known. Thus, 'a loaf of bread' is correct.

Abstract nouns like 'truth' take the definite article when used in a specific sense. Here, it refers to a particular truth about an incident. 'A truth' would suggest one among many, which is not intended. 'No article' would be incomplete. Therefore, 'the truth' is correct.