Ich brauche einen Begleiter. Ich hatte gehofft, dass er sich bereit erklärt, wenn er mich die Girlande flechten sieht, aber ...
= "I had hoped he would step forward for the role when he saw me weaving the garland ..."
I have already come across several instances of this apparent tense-mismatch, but if an equivalent English sentence is anything to go by, I wonder why German prefers to express the idea in a subordinate clause in the present tense even though you are obviously talking about what has already happened in the past.
Is it simply ungrammatical to use other tenses than the present in this subordinate clause?