Question

how do you know when to use La or UNA

can you give me a clear distinction between the two.

Answer 1

Whenever you would use the word "the", the so-called "definite article" in English, you would use "la" in Spanish, assuming that it goes with a feminine noun.

Whenever you would use the "indefinite article "a" or "an" in English, you would use "una" (feminine) or "un" (masculine) in Spanish.

Answer 2

Sam, you asked the same question the other evening about "las" versus "unas" and I explained it at that time.

Indefinite article = A, AN = un, una, unos, unas

Definite article = THE = el, la, los, las

Look back to find that! Of course drwls is correct that it has to do with the number (singular/plural) and gender (masculine/feminine) of the noun

el or un libro
la or una casa
los or unos libros
las or unas casas

Sra