Is there a rule for unwritten a?

I wonder where an unwritten a is used and where not. There are cases where it would influence the tone of of the second syllable like in สะพาน (bridge) and สว่าง (bright), but in ระวัง (to be careful) and สะดวก (convenient) it doesn’t. Here some more examples:, อร่อย, อะไร, พัทยา (one a is written, the other is unwritten), ละคร (play ; stage performance ; drama) , นคร (city), ทวีป (tá-wêep continent), ทะเล (sea, ocean).

Can you see any regularity among the cited examples?

How do you memerise words with written or unwritten a?

There are a few guiding principles. The rules, in order of priority, seem to be:

1. The vowel is written in closed syllables, either by อั or รร.
   
2. With a very few exceptions (e.g. พระพุทธเจ้า), word-final /a/ is always written. This used not to be the case, and does not apply within compound words, e.g. สัตวแพทย์.
   
3. ะ is only inserted within Pali and Sanskrit loanwords in the Sanskrit(ised) prefix ประ. (I'm open to correction on Sanskrit word-internal visargas.)
   
4. ะ is not used if its consonant affects the tone of the following syllable.
   
5. There is a tendency not to use it to split consonant clusters from other languages, e.g. แสดง, สไปรท์.
   

The written and unwritten 'a' are different vowels. The written form is pronounced with a glottal stop after it and takes a high or low tone according to the consonant class. The unwritten form is not followed by a glottal stop, and in all but the most formal speech is unstressed and approximates to a mid tone.