Learn Arabic (Modern Standard): العود أقدم من الجيتار.

To say 'older', Arabic uses a specific pattern for comparatives. You take the root letters of the adjective—in this case, ق-د-م from قديم (old)—and put them into the pattern أَفْعَل (af'al). This gives us أقدم (older).

Does من usually mean 'from'? Why is it translated as 'than'?

Yes, من (min) normally means 'from'. However, when it is used immediately after a comparative adjective like أقدم, it acts as the word 'than'. Together, أقدم من literally translates to 'older from' but means 'older than' in English.

Why is 'guitar' spelled with a ج (jeem) in الجيتار?

Modern Standard Arabic does not have a native hard 'g' sound (like in 'good'). When adopting foreign words, Arabic usually replaces the 'g' sound with a غ (ghayn) or a ج (jeem). For 'guitar', the standard spelling is الجيتار, though you might hear it pronounced with a hard 'g' depending on the speaker's regional dialect.

Why does Arabic use the definite article ال on both instruments here?

In Arabic, when you make a broad statement about an entire category or class of things (like the oud or guitar as a general invention), you must use the definite article ال (al-). Even if we translate it generally into English as 'Ouds are older than guitars', Arabic requires the nouns to be definite.