Although stemming from the same kanji, まだ 'still' will almost always be read as 未（いま）だ 'still' when appearing in its kanji form. When combined with に, like まだ, 未（いま）だ will indicate that something is 'still (A)', or 'not yet (A)', depending on whether the verb it is paired with is negated or not.
As
未（いま）だに will be used adverbially, it will mostly appear at the beginning of sentences.
スマホが普及（ふきゅう）している中（なか）、未（いま）だにガラケーを使（つか）っている人（ひと）がいる。
While smartphones are becoming more and more popular, there are people who still use flip phones.
私（わたし）が子供（こども）の頃（ころ）に巻（ま）き込（こ）まれた事故（じこ）のことは未（いま）だに覚（おぼ）えている。
I still remember the accident that I was involved in when I was a child.
部長（ぶちょう）は人事（じんじ）に注意（ちゅうい）されて２ヶ月（かげつ）も経（た）つのに、彼（かれ）の態度（たいど）は未（いま）だに変（か）わらない。
It's been two months since the director was warned by HR and his attitude still hasn't changed.
去年（きょねん）行方不明（ゆくえふめい）になった少女（しょうじょ）は未（いま）だに発見（はっけん）されていない。
The girl who went missing last year has still not been found.
Compared to
まだ,
未（いま）だに is more formal, and will often imply that something has been continuing for a much longer period of time. This is similar to adding the word
ずっと to a standard sentence.
小学生（しょうがくせい）の頃（ころ）、高橋（たかはし）くんにいじめられたことはまだ根（ね）に持（も）っている。
I am still holding a grudge over the fact that I was bullied by Takahashi-kun when I was in elementary school.
小学生（しょうがくせい）の頃（ころ）、高橋（たかはし）くんにいじめられたことは未（いま）だに根（ね）に持（も）っている。
I am still holding a grudge over the fact that I was bullied by Takahashi-kun when I was in elementary school.