What is the difference when using dake and nara in sentences, and if possible an example sentence for each would be helpful too. Thank you very much. They both mean just if I'm not mistaken. Like if I said I only understand hiragana. ひらがなだけです. Or ひらがなならわかります. Which is better and why
I'm going to take a stab at this:
ひらがなだけです。 ([It's] only hiragana.)
ひらがなならわかります。 (If [it's] hiragana, [I] understand.)
They don't exactly mean the same thing, although they can carry the same connotation of the speaker wanting/needing ひらがな in order to understand the topic at hand (although the first phrase might be a little blunt and far-fetched).
Now to your actual question of differences between なら and だけ:
なら is, as @HdcLsh said, a conditional auxiliary. It's used in situations where there is an if-then relationship in the sentence:
If you read that book, you know this.
However, だけ is a separate particle, used to indicate "only" or "simply":
[I] only ate the apple.
Therefore, you can see the drastic difference in the actual definition of those words.