Telling the Time
Ask the time with Τι ώρα είναι; (“What time is it?”). Because η ώρα (“the hour”) is FEMININE, one o’clock takes the feminine μία, not the neuter counting form ένα: Είναι μία. From two o’clock on you agree with the feminine plural ώρες, so the indeclinable numbers stay put (Είναι δύο) but 3 and 4 take the masculine/feminine τρεις / τέσσερις, never the neuter τρία / τέσσερα (Είναι τρεις). To say “at” a time, reuse the σε-contractions: singular στη μία (σε + τη μία) but plural στις δύο / στις τρεις (σε + τις ώρες). 12-hour clock.
The rule
Ask with Τι ώρα είναι;. The grammar of the answer hangs on one fact: η ώρα is feminine, so the numbers agree with it and the neuter counting forms are wrong. One o'clock is Είναι μία - the feminine μία, not the neuter ένα. From two o'clock on you are agreeing with the feminine plural ώρες, so the indeclinable numbers simply stay as they are (Είναι δύο) but 3 and 4 take the masculine/feminine τρεις and τέσσερις, never the neuter τρία / τέσσερα. To say “at” a time, reuse the σε-contractions and mind the number: singular στη μία (σε + τη μία) but plural στις δύο, στις τρεις (σε + τις ώρες). The clock is a 12-hour one, with the part of the day added when it matters.