Why is it 'mets-les' but 'lave-les'
The S is removed from all ER verbs in the TU imperative! But is retained for all other verb forms. LAVER is an ER verb so we remove the S and say, "LAVE-LES". But METTRE is not an ER verb, so we keep the S and say, "METS-LES".
What a damn silly rule