For all levels: What are some true cognates and some false cognates between your native and target languages?
Because Welsh is so very different linguistically from English, you don't get the type of "false friends" that exist between say German and English where two words have the same root and still resemble one another but they have diverged in meaning. At least I can't think of any. However, there are words that can confuse English speakers when they're beginning to learn Welsh because they look or sound like English words but are completely different in meaning.
plant -- sounds exactly like the English word "plant" but means children.
union -- looks like the English "union" but is pronounced "eenion" and means exact.
pam -- sounds exactly like the English name "Pam" but means "why".
bob -- sounds similar to the name "Bob", but means "every".
Borrowed from the English but spelled the phonetic Welsh way. Far too numerous to list them all, but here are a few:
bws / bus
ambiwlans / ambulance
snwcer / snooker
bisged / biscuit
siocled / chocolate
Along the same lines, here are some verbs borrowed from English but made Welsh by adding "io" on the end.
sortio / to sort
licio / to like
joio / to enjoy
And finally there are the words that sound like the English word and mean the same but they didn't come from English. Both the English and Welsh words have a common root in Latin. In English, words with Latin roots tended to come via Norman French, but in Welsh it's more likely they came directly from the Latin when the Romans invaded Britain.
castell / castle (from the Latin castellum)
tafarn / tavern (taverna)
awr / hour (hora)
meddyg / medic (medicus)
person / person (persona)
sŵn / sound (sonus)
papur / paper (papyrus)