Learn Arabic (Modern Standard): القلم قديم.

In Arabic, simple statements like this are called nominal sentences. You do not need a word for 'is' in the present tense. By placing a definite subject (القلم, the pen) next to an indefinite descriptive word (قديم, old), it automatically forms the complete thought 'The pen is old.'

How do we know that قديم (old) is indefinite here?

It does not have the definite article ال (al-) at the beginning. If you were pronouncing the full grammatical endings, it would end with an 'un' sound (called Tanween). This indefinite form is used here because it acts as the predicate—the 'news' you are delivering about the pen.

What would it mean if both words started with ال (al-)?

If both words were definite, making it القلم القديم, it would no longer be a complete sentence. It would just be a descriptive phrase meaning 'the old pen.' To make a complete sentence ('The pen IS old'), the second word must remain indefinite.

Why do we use قديم instead of the feminine form قديمة?

The word القلم (the pen) is grammatically masculine because it does not end with the typical feminine marker, the Taa Marbootah (ة). In Arabic, the describing word must match the gender of the noun, so it stays in its basic masculine form.