Translation:You should write
Mu denotes you (plural) as the subject in the subjunctive e.g. 'Muandike' - 'you (pl) ought to write'
Note: this only applies to verb stems that begin with vowels, otherwise the prefix is just M e.g. 'Msome' - 'you ought to read'
Mw denotes him/her as the object in the imperative e.g. 'Mwandikie' - 'Write for/to him/her'