"I read the book with Gareth."
Translation:Darllenais i'r llyfr efo Gareth.
Is "dw i'n darllen y llyfr efo gareth" also acceptable here - in english the present and past tense of read cannot be discriminated in this sentence.
For Tree2 we will add a definite indicator of the past to the phrase- ddoe or similar. In the meantime, it is best to rely on this being in a section specifically dealing with the past tense to indicate that you are being asked to practise the simple past pattern.