"I eat this chicken."
Translation:Tôi ăn con gà này.
đây is used as a subject of a sentence (ex. THIS is ...), so it's common you would see đây là together ...
đây là chiếc xe của tôi (translation: this is my car)
đây là rất khó ̣(translation: this is too difficult)
Meanwhile, này is used as an object (ex. THIS car, THIS machine, THIS gate, THIS + any noun). Just remember này is placed after the noun when translating it to Vietnamese.
Tôi thích máy này. (translation: I like this machine)
Máy bay này bay xa. (translation: This airplane flies far ... literally: this flying machine flies far)
Cậu bé này muốn ăn cơm. (translation: This boy wants to eat rice)
Reminder: con is 90% alive and cái only 10% ...(yes, there are always exceptions). Basically, Vietnamese has three general classifiers (cái for inanimate objects, người for people and con for non-human beings) and about two hundred more specific classifiers I will not list in here now ;-)