Is "da" used often in the casual speech?


Is "da" used often in the casual speech? Or is it often omitted whenever it's
possible? For example:

1) Genki (da)?  
2) Suki (da)?  
3) Ke wa atsui (da) ne?




It is **difficult to give a precise answer** to this question. In cases where
the speaker has a choice between "da" and just ending the sentence, both have
their own nuances. Omission may be more "feminine" and addition of da might be
more "masculine". In some cases, da can be used for emphasis. **Usage patterns
vary by gender, age, social situation, and possibly by region/dialect** as
well.

There are **many other casual sentence enders** besides just "da"/""
(nothing). You'll also hear stuff like "genki na no?" -> "genki da yo".
There's also "kai" and "dai" for questions, "wa", "jya" and many others. In
fact, **sometimes personal choice of sentence enders is used to "project" a
certain "personality type"**. It is something you **develop an "ear for"** the
more you speak to a variety of people.

Note that there are grammatical limitations, however. For example, as per
Yuuichi Tam's comment, you wouldn't use "da" as a question end. For a
question, you can replace "da" with "ka" (but this can sound sharp or rude in
some cases, so be careful) eg "genki ka?", or just end the sentence with
rising intonation "genki?". There's also other question forms like "___ (na)
no (ka)?" eg "genki na no?" but that's a more advanced topic.

About the example sentences:  
As statements, all three would be okay with da or without. As questions, the
first two are wrong if you use "da". The third one sounds weird to me but is
close.. if you said something like "atsui da yo ne?" that would probably be
okay, but that's because it is more like a statement followed by a request for
confirmation: "atsui da yo" (statement) + "ne?" (question)

