Learn Russian: Добавленный мастером штрих полностью меняет заглавную букву. - A stroke added by the master completely changes the capital letter.

This directly relates to the punctuation rules for participles. When a participial phrase (a participle and its dependent words) comes directly before the noun it describes (штрих), it is not set off by commas. If we moved it after the noun, we would need commas: Штрих, добавленный мастером, полностью меняет...

Why is the word мастером in the instrumental case?

Добавленный is a past passive participle meaning "added." In Russian, when you use a passive voice construction, the "doer" of the action (the agent) takes the instrumental case. Here, мастером translates to "by the master."

Why is the participle добавленный perfective but the main verb меняет is imperfective?

They describe actions happening at different times. The perfective participle добавленный shows that the adding of the stroke is a completed, past action. The main verb меняет (changes) is present imperfective because it describes an ongoing result or a general fact: that previously added stroke continuously alters how the letter looks.

Is it more natural to say заглавная буква or большая буква for "capital letter"?

Both are extremely common. Большая буква is what you will hear most often in everyday conversational speech. Заглавная буква is the proper typographical term, making it slightly more formal. This fits perfectly with the context of a "master" (likely a calligrapher or artist) altering the letter.