Learn Russian: Написанные чернилами штрихи удивительны.

The participial phrase Написанные чернилами (written in ink) comes directly before the noun it describes, штрихи (strokes). In Russian, when a participial phrase precedes its noun, you do not use commas. Commas are only required when the phrase comes after the noun.

How would the punctuation change if we moved the participial phrase after the noun?

If you put the phrase after the noun, it must be enclosed by commas: Штрихи, написанные чернилами, удивительны. The meaning remains exactly the same, but placing the participial phrase after the noun is very common in descriptive writing.

Why is the word чернилами in the instrumental case?

Чернилами is the instrumental plural form of чернила (ink). Passive participles like написанные (written) frequently take an instrumental noun to indicate the tool, material, or agent used to perform the action. Here, it means written with or in ink.

Why does the sentence end with удивительны rather than the regular adjective удивительные?

Удивительны is a short-form adjective. Short forms are commonly used when the adjective acts as the predicate of the sentence (stating what the strokes are). Using the short form here makes the statement sound more definitive and literary, which fits the tone of the sentence.