If I understood correctly, both 完 and 了 can be used to indicate a completed action, so what is the difference in use? How do we properly use 完 in a sentence? Which contexts would be valid?
狗 (gǒu) and 犬 (quǎn) both mean "dog" and both seem relatively common, but is there any difference between them? When should I use one and when should I use the other?