How would this sentence be different if I wanted to refer to a specific breakfast (e.g., a scheduled one)? "We meet him during THE breakfast."
You can only know by context or by describing it more specifically (e.g. het ontbijt van morgen), the sentence in the exercise can mean both some specific breakfast or breakfast in general. Putting emphasis on hét doesn't really work since this implies it is some important or remarkable breakfast.
If you wanted to say "We meet with him," would it be "Wij ontmoeten met/bij hem"?
It would just be 'wij ontmoeten hem'. 'ontmoeten bij' only indicates the place, for example: 'we ontmoeten elkaar bij het café' (we('ll) meet at the café)
Could this sentence also be translated as "We met him during breakfast"? That is to say, is the present tense of "ontmoeten" the same as the past tense?
No, though both tenses sound the same. 'Ontmoeten' in the past tense would be 'ontmoetten' with two t's. The rule here is that (with regular verbs) you take the infinitive (ontmoeten) and you take away 'en' at the end of the word so you keep the stem (here: ontmoet). Then you add 'te' or 'de', depending on whether the last letter of the stem is one of the letters tkfschp (mnemonic is ' 't kofschip', but without the vowels). If it is one of those letters, you add 'te' (or 'ten' if it's plural). If it's not one of those letters, you add 'de' or 'den'. The last letter here is a 't', which is one of those letters, so you have to add 'ten' to ontmoet, which makes the double 't'.
Another example would be 'borstelen' (to brush). -en = borstel. 'l' isn't one of the letters in 't kofschip, so the past tense is borstelde (singluar) or borstelden (plural). Hope this helped!
Does "we're meeting him for breakfast" mean something different from this sentence?
When you meet someone "for" breakfast it means you made plans to do so. When you meet someone "during" breakfast it means you meet them while you're having breakfast, unexpectedly.
Waarom wordt "ontmoetten" fout gerekend? Je kunt toch niet aan de uitspraak horen of het tegenwoordige of verleden tijd is.