A combination of two letters representing one sound is called digraph (dwuznak, plural: dwuznaki). There are few of these in Polish language.
Here they are as well as examples of words we use them in:
CH – In modern standard Polish, “ch” is pronounced identically to “h”. It obviously makes the pronunciation aspect easier while messing up the simplicity of the spelling at the same time… For most Polish students, learning when to use “ch” and “h” is a painful process involving hours of writing mind-bogglingly boring dictations. The same goes for rz and ż, and for u and ó.
CZ – This sound is pronounced similarly to the English ch in chalk
czytać (to read)
czuć (to feel)
DZ – it becomes softer sound when followed by “i” (for example “dzik” – boar)
DŹ – Try not to confuse it with ć or dż!
dźwig (lift, elevator)
DŻ – It sounds more or less like the English j in Joe.
RZ – You pronounce it identically to ż – practically the same as the s in leisure or pleasure.
SZ – similar to English “sh” in shovel
szarlotka – (apple pie)
Let me know if you have questions:)
Do następnego razu… (Till next time…)