Learn Polish: Jego serce bije teraz szybciej. - His heart is beating faster now.

The basic adverb is szybko (fast or quickly). To make it comparative (meaning faster), we drop the -o and add the ending -iej. Notice that the k changes to a c for easier pronunciation, making it szybciej.

Does the verb bije only apply to a heartbeat?

No, bije comes from the verb bić, which means to hit, to strike, or to beat in a general sense. It is used for physical hitting, a clock striking the hour, as well as a heart beating.

Serce is a neuter noun. Why doesn't jego change its ending to match it?

Unlike words like mój (my) or twój (your) which change endings to match the noun's gender, third-person words like jego (his), jej (her), and ich (their) are frozen. They always stay exactly the same, no matter what noun comes after them.

Why is teraz placed before szybciej, unlike the English translation?

Polish often puts the most important piece of new information at the very end of the sentence for emphasis. Because the main point here is how the heart is beating (szybciej), it naturally takes the final spot.