Why are ehters cleaved by acids and not by bases or alkalis?

This is because hydronium ions in acid attacks the lone pair on oxygen and a positive charge develops there. Any charged substance is unstable so they get cleaved to come to a stable position. With bases this cant happen thats y. 

ethers hv formula R--O--R^'. see that nucleophiles and bases hv no place to attack on ethers which may be favourable to them as they contain negative charges or source of electrons. bcoz there is already O with its lone pair so it can be attacted by a positive centre(eletrophile or acid) or donate its lone pair.

so ethers r generally less reactive(only with some acids) and resist attact by bases n nucleophiles.

they r good solvents as they solvate cations of the material by donating lone pair of oxygen and it ets hydrolysed.

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