key words are simply common words which continuously crop up in texts
They are indeed, and remain so irrespective of the latest teaching fashion or fad. Letters and Sounds lists 300 of them, but the first 100 of them are the most high frequency of all.
Many of them have phonically regular spellings (a, and, at, in, it), but 40 among the first 100 are tricky in various ways:
the - he, be, we, me, she,
was, want, all, call, said,
of, to, one, come, do, down, into, look, now,
only, other, some, two,
could, you, your,
when, what, where, which, who, why,
there, were,
right, are, have, before, more.
Phonics fanatics claim that they teach those by the phonic method too, but they have to keep going over them so many times, because most children keep stumbling over them for a long time, that children may well just be imprinting them on their minds anyway.
They are clearly trickier to learn than the 190 of the 300 English most HF words with phonically consistent spellings:
a, am, an, and, as, at, back, bad, can, cat, dad, gran, grandad, had, has, hat, man, rabbit, ran, sat, than, that, that?s,
came, gave, made, make, place, take, same, name, baby,
away, day, may, play, say, way,
car, dark, garden, hard, park,
bed, best, better, eggs, end, fell, get, help, let, let?s, next, red, tell, them, then, very, well, went, yes, her,
been, feet, green, keep, need, queen, see, sleep, three, tree, trees,
each, eat, sea, tea, please, even, here, these,
big, children, did, didn?t, different, fish, him, his, if, in, is, it, it?s, its, king, little, miss, still, thing, things, think, this, will, wind, wish, with,
birds, first, girl,
inside, like, liked, time, I, I?ve, cried, night, right, by, fly, my,
box, dog, fox, from, got, hot, long, lots, no, not, of, off, on, so, stop, stopped, top, floppy, across, along,
cold, old, told,
go, going, home, over, clothes,
or, for, horse, morning,
found, house, mouse, our, out, round, around, shouted, about, boy,
but, duck, fun, just, much, mum, must, run, sun, under, up, us, jumped, suddenly,
use,
their, they, new, again, air, because, began, boat, window.