Here are some music sheets to get you more used to the challenges of left hand sight reading. This would be good for people playing Grade 5 music and above. Left hand sight reading is lots about knowing what types of chords to expect in a piece in any given key. So, work out the key signature first, work out what the names of chords 1, 4 and 5 are, and then check out what rhythms appear in the piece. Then it's time to have a go.
The first sheet has notation that looks a bit small, so maybe try a different one first.
I'll put up exercise no. 2 as soon as I can.
Left hand practice no. 1
Left hand practice no. 3
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