The head teacher has ____ nothing so far about the proposed timetable.

Explicación

Has opens a present perfect, so the verb must appear as a past participle, and this verb keeps just one past form for that slot. The same word covers the plain past too: she said so on Monday and she has said so twice differ only in the auxiliary, not in the verb shape. Sayed regularises a verb that changes its vowel and takes -d, and says is a present-tense form that cannot follow has.