The main thinking strategy for the +9 facts is ‘add 10’, and then take away 1. For this reason, this strategy should be introduced after the +10 facts and after students have investigated teen numbers.
Another key subskill is knowing the ‘one less’ facts so the students can efficiently back up one from their total. The +1/-1 numbers need to be extended into the teen numbers. One tool for doing this is to use Teen Number Flash cards. After students are able to identify the teen number within a couple of seconds, start asking for one more, one less, two more, and two less than the amount shown.
The +8 facts are very similar, except students need to take 2 away after adding ten.
Adding 8 or 9 is essentially the same as as the Make 10 facts, which are most efficient when the facts are 8 + ? or 9 + ?. Using the commutative property, these could be approached with that strategy instead.