Is だ informal for です?
I was watching anime and noticed they use a lot of だ(da) instead where they should use です(desu). So is da like informal for desu?
Sometimes it replaces です in a casual sentence but it is not the same. だ is affirmative so it can't be used to in a question ie そうですか cannot become そだか because you can't ask a question and be affirmative at the same time.
The last section of the page on this link describes it in more detail: http://www.guidetojapanese.org/learn/grammar/polite
It is my understanding that you would not pair だ with か because they are different formality/politeness levels. I have definitely come across examples of people using だ to form a question in casual speech, especially with male speakers.
I believe it is allowed, grammatically, but sounds pretty rough or impolite.
For a long time I used to think か is used only after the 連体形 of the verb and unable to be used after the 終止形. I also used to think だ was a contraction of only the 終止形 of the copula (contraction of the old sentence ending form であり, rather than a contraction of である).
Because I thought these two things, I saw this as the reason why か is unable to be used after だ.
I can't really remember why I stopped thinking all this. It's been a long while since I last spent time on Japanese grammar. I've now even forgotten whether my understanding actually changed about this. ^^;
Short answer: yes, roughly but not exactly
Longer answer: (according to some notes from a class I took years ago) だ can follow a noun-like word or a phrase particle, but not an adjective-like word. It's also frequently dropped in a number of contexts such as final sentence position or before most sentence particles (か、ね、よ, etc) unless it's past tense (だった). だ is also used mid-sentence for independant clauses in a more complicated sentence whereas です is only used at the end (ok, です can be followed by the particles, but it's mostly at the end)
本当か？ or 本当ですか
今日行きますか？今日行くの？ or 今日行くかい？
There are lots of variations on things like this in the informal, used between friends, and it depends on the social situation. I worked with crews of guys on international construction projects in Western Africa and they had a rough vocab that wasn't recommended back in the office back in Tokyo.
I think this is where watching movies and reading books and manga helps a lot.
That's true. It definitely depends on your "audience". I mean there are social groups in the U.S where "where you at" is a common phrase even though it's not technically proper grammar.