Chinese characters are logograms - written characters that represent a word of phrase.
The logogram for qigong is:
氣 功
Even though there are two characters, they represent a single word.
The problem is in converting the above into characters of our alphabet - transliteration.
Up until the 1970s the most popular way of doing this was Wade-Giles. Under that system, it was written as
ch'i kung, simplified to
chi kung.
The way Tai Chi (
太極拳) is written is also based on Wade-Giles (t‘ai chi ch‘üan). This was simplified to
tai chi chuan and abbreviated by most English speakers as
tai chi.
Wade-Giles has since (in the 1980s) been replaced by
pinyin - the system used by Chinese people today, governments, and international organizations for transcribing Chinese characters into Western writing.
With pinyin, what was written as
chi kung using Wade-Giles is written as
qigong.
Tai Chi is written as
taijiquan using pinyin, but "tai chi" is so firmly entrenched that taijiquan is unrecognizable to most people and is far more "searchable" on the Internet because so many articles use the Wade-Giles spelling.
Chi Gong (in the forum name) appears to be a hybridized version of the Wade-Giles and pinyin systems - Chi from Chi Kung and Gong from qigong.
Something that adds to the confusion is the "chi" in both chi kung and tai chi under Wade-Giles. While they share the same English characters under that system, the words have different meanings and pronounciations.
The "chi" in chi kung comes from the logogram
氣 and it means air, breath, or vital energy.
The "chi" in tai chi comes from the logogram
極 and it means "ultimate" but when combined with the character for tai 太, it's commonly translated as supreme ultimate or cosmos.
The way the words are pronounced do not follow normal English pronunciation rules. Here's a video on
how Qigong is pronounced. The closest phonetic way I can think of spelling it is "chee gung" (not "key gong").
For fun, here's
Bruce Lee pronouncing taijiquan. This
documentary has other people pronouncing it. Phonetic spelling - "tie gee chwan" or "tai gee" for the abbreviated name.
So the correct, modern way of spelling the words are qigong and taijiquan, but if you want anyone to understand you or you need to search for information online, you'll probably need to stick with "tai chi."