Learn Turkish: Hoş bulduk, teşekkürler.

Literally, hoş bulduk means "we found [it] pleasant." Hoş means "nice" or "pleasant," and bulduk is the past tense of bulmak (to find) with the "we" ending. It is the standard, fixed reply whenever someone welcomes you with Hoş geldin or Hoş geldiniz.

Can I say hoş buldum if I am by myself, instead of using the "we" form bulduk?

Yes, you can say hoş buldum ("I found it pleasant") when you are alone. However, hoş bulduk has become such a fixed expression that many Turkish speakers still use the "we" form even when they are by themselves! Both are perfectly fine to use.

Why does teşekkürler have a plural ending?

The word teşekkür means "thank" or "gratitude," and adding the plural suffix -ler turns it into "thanks." It is very similar to how English speakers say "thanks" instead of "thank you." It is slightly more casual than the full phrase teşekkür ederim.