I faced a problem with using "a" befofe singular nouns

hello there,
actually this is the first post for me here in English forum ;P

I was trying to learn a grammar about using: a, an, the from Dar Alakkad book in page 132 , and I found three cases we should use -a- in sentences. the cases are:

1- singular countable nouns
such as : a man , a book , an apple ... elc

2- before a singular job
such as: a doctor , a teacher ....elc

3- in expressions with numbers to mean every
such as: once a week , four times a yeat ... elc


the problem is:
when I tried to answer exercise 19 on page 135 I found other cases we can use a, an with! such as:

question 9: Omanis usually drink a little coffee after eating dates.

here we used -a- in another cases(none of the three cases on page 132)

also in question 11: Dad likes to go for a walk in the morning.

here also another case!! so now im really confused! how can I understand this! please help me :wacko:

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لمتابعة أكثر خارج موقع مسندم.نت...
نرجو زيارة هذا الرابط (( بالضغط هنـــا )) ..