Negation, emphasis, conditionals, and the one-letter words that attach to others.
Arabic doesn't negate every tense the same way; the particle itself tells you when the action happens.
لَمْ is followed by a present-tense verb even though it makes the meaning past — لَمْ يَلِدْ "He did not beget".
Six particles that open a sentence and put the following noun in the accusative, each adding its own shade of meaning.
The rest of the common particles, grouped by the job they do.
Conditional — إِنْ (in, "if", open) · لَوْ (law, "if", contrary-to-fact) · لَوْلَآ (lawlā, "were it not for")
Answering — نَعَمْ (naʿam, "yes") · بَلَىٰ (balā, "yes" — specifically answering a negative question) · كَلَّا (kallā, "no, never!")
Connecting — بَلْ (bal, "rather") · لَٰكِنْ (lākin, "but") · أَمَّا (ammā, "as for") · إِذًا (idhan, "in that case")
These are never written on their own — they attach to the front of the next word.
Several can stack: وَبِٱلْحَقِّ is وَ + بِ + ٱل + حَقّ — "and with the truth".