"He runs and she runs. They run."
Translation:Ritheann sé agus ritheann sí. Ritheann siad.
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In this case, yes:
Ritheann sí. = She runs.
Ritheann siad. = They run.
But not in every case; for example, to tell or command an individual person to run:
Rith! = Run!
To see a complete conjugation for this verb, go here:
scroll down to the entry for 'rith verb,' click on the word 'verb' just to the right of 'rith,' and explore the conjugations enumerated in the window that pops up.