Already means by now. All ready means completely prepared. The booklet was already open. The students were all ready. Diction as a set is in word choice.
Time versus prepared
If you can swap in by this time, use already. If you can swap in fully prepared, use all ready. One word versus two is the whole test.
Adapt versus adopt is another look-alike pair, in adapt versus adopt. Then versus than is in then versus than. Grammar overall is in ACT grammar rules.
The bottom line
By now: already. Fully prepared: all ready.
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