ona vs ni
How to use ona and ni and their differences:
ona is a third person pronoun, similar to English's it, they, she, him, themselves, etc.
ona can contain the information of a noun phrase (a head noun plus its modifiers), here are some examples:
mi kepeken ilo ona li ponami kepeken ilo. ona li pona.
I'm using a tool. It [the tool] is good.
jan [moku insa mi ijo] li moku e kili onajan Mimi li moku e kili ona.
Mimi is eating her [Mimi's] fruit.
kasi mute li lon ona laso li ponakasi mute li lon. ona laso li pona.
There are many plants. The blue ones [plants] are good.
ni is a demonstrative pronoun, similar to English's this, that, these, and those.
Besides being able to contain the information of a noun phrase, it can also hold the information of an entire sentence. Not only that, the information that ni contains can come after it is said, examples:
mi kepeken ilo ni li ponami kepeken ilo. ni li pona.
I'm using a tool. This [using a tool] is good.
jan [moku insa mi ijo] li moku e kili ni> ona li loje walo li tan ma [pilin awen sona insa jan uta]jan Mimi li moku e kili ni: ona li loje walo li tan ma Pasiju.
Mimi is eating this [information will come] fruit: it [the fruit] is whitish red and it comes from Brazil.
mi wile kama sona e toki Alapi mi o nimi wile kama sona e toki Alapi. mi o ni.
I want to learn the Arabic language. I should do this [learn the Arabic language].
Note that when ona is used, [ijo] ni can be used instead:
mi moku e kili ona li ponami moku e kili. ona li pona.
I eat a fruit. It [the fruit] is good.
mi moku e kili kili ni li ponami moku e kili. kili ni li pona.
I eat a fruit. This fruit is good.
Notice in the first example of each (mi kepeken ilo), how substituting ona for ni changes the meaning of the second part. Here are more examples of the difference ona and ni can make in a sentence:
mi wile ala moku e kili onami wile ala moku e kili ona.
I don't want to eat her fruit.
mi wile ala moku e kili nimi wile ala moku e kili ni.
I don't want to eat this fruit.
mi sona e toki mute ona li ponami sona e toki mute. ona li pona.
I know many languages. They [the languages] are good.
mi sona e toki mute ni li ponami sona e toki mute. ni li pona.
I know many languages. This [knowing many languages] is good.