Studying /ˈstʌdiɪŋ/ is the -ing form of study. Keep the y before -ing: verbs ending in consonant + y never change it. The word has three syllables: STUD-ee-ing.
I'm studying English tonight. Note: studing is a misspelling, and studding is a different word entirely.
She is studying for her exam this weekend. A natural use in the present continuous, pronounced /ˈstʌdiɪŋ/, roughly STUD-ee-ing.
We spent all night studying the map. Here spent is the simple past of spend, and studying works as an -ing complement after it. Even so, the spelling point holds: the base verb's d is not doubled, so no extra d appears before -ing.
One trap: only studies, the third-person singular, changes y to i. Studys is not a spelling. The base stays study, so the -ing form is simply study + ing.
The transcription /ˈstʌdiɪŋ/ has three syllables: STUD-ee-ing. The stressed first syllable is /ˈstʌd/ with a short u, and the second syllable carries the ee-like vowel /iː/ from the letter y, so the middle sounds like stud-ee. The final unstressed syllable is /ɪŋ/, a short /ɪ/ followed by /ŋ/. This is where the printed letters ee can mislead: they appear only in the spelling, not in the final syllable, which stays light and short.