Learn German: Morgen ist der dreißigste Geburtstag von Tom.

To say 'thirtieth' instead of 'thirty' (dreißig), we use an ordinal number. In German, numbers from 20 onwards add -ste (or -sten) to the base number. This is a slight shift from smaller numbers, which just add -te (like der neunte for 'the ninth').

Why does it end in -e (dreißigste) instead of -en (dreißigsten)?

Ordinal numbers take the exact same endings as regular adjectives. Because Geburtstag is a masculine noun and is the subject of this sentence (nominative case), the ending after the definite article der is simply -e. You will see -en in other situations, such as saying 'on the 30th birthday' (am dreißigsten Geburtstag).

Could I say Toms dreißigster Geburtstag instead of using von Tom?

Yes, you can! Saying Toms dreißigster Geburtstag is perfectly correct. However, using von ('of') plus the name—literally 'the 30th birthday of Tom'—is a very common and natural alternative in everyday spoken German to show possession.

Why does the verb ist come before the subject?