Learn Arabic (Modern Standard): المدير لم يقل إن المرض عذر مقبول. - The manager did not say that illness is an acceptable excuse.

Because it follows the negative word لم (did not), which puts the present tense verb into the Jussive mood. For hollow verbs like قال (to say), where the middle root letter is a long vowel (the waw in يقول), the Jussive mood drops the final short vowel. This leaves two unvoweled consonants in a row. To fix this, Arabic shortens the word by dropping the long vowel entirely, giving us يقل (yaqul).

Why do we use إن (inna) for "that" here, rather than أن (anna)?

Normally, you use أن (anna) to say "that" in the middle of a sentence (like "I think that..."). However, Arabic has a strict rule: after the verb "to say" (like يقل here), you must always use إن (inna) instead.

If I want to pronounce the case endings, what ending does المرض (illness) take?

It takes an accusative ending, which is a fatha (an "a" sound), making it المرضَ (al-maraḍa). This is because it immediately follows إن (inna), which forces the noun right after it into the accusative case.

Why doesn't the Arabic sentence use a word for "an" in "an acceptable excuse"?

Arabic doesn't have an indefinite article like "a" or "an". Simply writing the noun without ال (al-), as in عذر (excuse), already makes it indefinite. The adjective مقبول (acceptable) matches it by also dropping the ال.