Why isn’t “That is a big cow.” accepted? Because it lacks एक ?
Precisely. The indefinite article ‘a’ is implicit and not required only in a few cases, for example, in sentence constructions like “I eat a [—-]”. However, in sentences like this one (which have the format “there is a [—-]”), एक is required.
Why 'badi' and not 'bada' ?
That's because cow is feminine, every adjective for a female ends with the syllable 'i'
Thy hey will com
For बड़ा, it had 'large' as a possible translation. It does not for बड़ी and did not accept "the cow is large" an an answer. Is "large" not a valid translation?