Focus, topic, cleft

kú, bï, and nä are three particles that serve closely related pragmatic functions. Let’s examine the ways in which they differ.

Main use

Báq shamu bï, mí Laqme nä soqcho hóa kú báq soe.
As for apples, Laqme, she likes sour ones best.

In this sentence…

bï is a topic marker. It identifies what the sentence is about. Here, it tells the listener that we're making some point about apples.
nä is a cleft verb. It purely helps us rearrange words when we feel like it. Here, it achieves a pseudo SVO word order.
kú is a focus marker. It identifies the new or contrasting information in the sentence: What apples does Laqme like? Sour apples.

The syntax of bï is: topıc bï comment.
- topic must be a definite noun phrase, like the cat or that I love you. If you want to refer back to the topic, you may not use hóa, but you may use any of the other strategies available to you, like máq or hụ́ꝡa.
The word nä can be used to front noun phrases and adverbials.
- When fronting a noun phrase, the syntax is noun phrase nä relatıve clause. The relative clause must refer back to the noun phrase, either using hóa or an appropriate pronoun like hó.
- When fronting an adverbial, the syntax is adverbıal nä clause. In this usage, hóa is not used because there’s nothing to refer back to.
The word kú goes in front of the focused piece of information. To focus a single word instead of a whole constituent, you can use the prefix form ku-. See Simple Focus.

Focus is often rendered as an English cleft in translation: Shao paı jí kú súq It’s you who I want to be friends with. However, Toaq clefts do not impart focus: Súq nä shao paı jí hóa is just another way of saying Shao paı jí súq. You may, however, use the two features in tandem: Kú súq nä shao paı jí hóa.
For noun phrases, bï does not bind hóa. You must use another anaphoric pronoun if you want to refer back to the topic. The crucial difference is that bï’s complement, the topic established, is not required to appear in the clause! So for example, this is valid Toaq (and valid English, although some languages like Japanese like this pattern better than English does) and the extra specification in square brackets is not required at all:
Báq kası bï, he duq garabıa jí [gêm máq].

As for walnuts, I tend to get nausea [from them].