Devanagari Letter Ka

Its default reading is ka, not just k.

Hindi ka uses the same क as Sanskrit. Sanskrit pronounces the built-in vowel unless a mark changes or cancels it. Hindi can leave that vowel unspoken within or at the end of a word. Marathi and Nepali have different patterns too: check the whole word in its language.

Why does क change shape?

Added marks change its vowel or join it to another consonant.

A vowel mark changes the syllable, as in का or कि. A virama, also called halant, cancels the built-in vowel. When another consonant follows that mark, the font may draw a reduced “half form” or a joined shape called a conjunct.

Where did क come from?

It evolved from an ancestor in the ancient Brahmi script.

Brahmi recorded language in inscriptions, including King Ashoka's public decrees from the third century BCE. The ancestor of क changed through later scripts; the northern writing tradition called Siddhamatrika eventually led to Devanagari. This was gradual development, not a single invention of today's shape.