Introduction
إلا ما often modifies a clause containing the word “once” “one”, or “a”. In the example sentences, I will put these words in green so you can notice them.
This structure is used when the speaker wants to emphasize the negative word "not" or "never". This is usually done in English by moving the negative word to the front of the sentence and then adding the word 'even'. I have put the word 'even' in some of the English translations to show that this meaning can be conveyed in Arabic without the word حتى "even". However, for more emphasis "حتى" may be inserted (see Examples 7 and 8).
You may be tempted to start this clause with و “and”; however, this is not attested in the examples in the corpus CALM.
The ما in إلا ما is a negator but the ش of negation is typically left off as it is with the ما of عمر ما (have never) and the ما used when one swears (يحلف) as in والله ما شفتك “I swear I didn't see you”.
Do not confuse this structure with the إلا ما that means "except + verb". (An example of this is given at the end of this page in Example 11.)