Always / never
Always split Aces and 8s. Never split 10s (or face cards) or 5s. Splitting 5s throws away a strong double; splitting 10s breaks a winning 20.
The middle pairs
2s, 3s, 6s, 7s: split vs weak dealers (exact chart depends on the pair). 9s: split vs 2–9 except 7. 4s: rarely split (only vs 5–6 with DAS). Learn the chart patterns; the quiz will lock them in.