I am struggling with that "一, yi1, one"-issue. Is there an exact rule, when the tonemarks have to be changed and when not? e.g.:
yi2 ge4 (an [apple or so]) < tone mark changes to second tone, because "yi1" is followed by a fourth tone.
yi4 dian3 (one O'clock) < tone mark changes to fourth tone, because it is followed by a first, second or third tone.
In case of numbers, the tone mark should not change, it remains first tone.
But what about January (yi1 yue4)?
And what about "yi1" is followed by a toneless tone?
I am confused.