When do you use अगले and अगला?
There is another aspect to it: in some cases, it is the oblique case although no postposition is present in the sentence: the postposition is implied. See these comments: https://forum.duolingo.com/comment/28369697
Like अगला लड़का। अगले लड़के। अगली लडक़ी। अगली लड़कियों
अगला अगले अगली are used with singular masculine, plural masculine and feminine (singular and plural) words respectively
Mnemonic for सोमवार = Monday: सोमवार has a [m] sound in it, like English "Monday".
But मंगलवार (Tuesday) also has an M sound & even begings with M! :)