Part 1b.2c.ii - The divisions: The four actions of speech

As for the four of speech:

Speaking false words is lying to change another’s opinion.
Related is speaking true words so that others receive them falsely.
Slander is speaking words that bring about dissension.
Related is saying one thing here, another there.

Idle talk consists of bad teachings and frivolous words.
Related is disconnected and other improper speech.
Harsh language is abusive words that denigrate others.
Related is using gentle words to make others unhappy.

Actions of speech are explained to make speech a gate of entering the Dharma. To speak words that are not true to change the opinions of others is false speech. Related to that is if one sees that another is deceived by literally true words.

Saying slanderous words that bring dissension to others is divisive speech. To say one thing to one person, and something else to another is related.

To start, spread, and listen to gossip; and to talk disconnectedly and frivolously, saying various things that contradict Dharma is idle or sophistic speech. To say something inappropriate for the time is related.

Harsh language is saying things unpleasant to the ears of others and insulting them. Related is saying things gently to make them unhappy.

The Abhidharmakosha says:

False words change the perception of another person
By their understanding of the meaning of the words.
Slanderous words are those that bring dissent to others,
These are the words of a mind that is possessed by the kleshas.
Harsh words are those that are unpleasant when they are heard.
All those with the kleshas are words of idle talk.

The commentary says:

True words that are deceptive, to repeat such words, to speak inappropriately for the time, and to speak pleasantly, but make others unhappy are the related actions.