The past perfect tells us that the action happened before the main past event. So: Until he drove his bike to my house, I hadn't noticed that he could ride a motorcycle now. (the time when you didn't notice is before he drove the bike). The past simple is the most common past tense and is used to talk about consecutive past actions or completed actions which don't need to be situated in time relative to other actions. When I saw him, I didn't notice that he could ride a motorbike. Later he told me that he could. (These happen consecutively).

Hi Dolco, To answer your last question, the past perfect tense does have its use when we are comparing the sequence of two past actions or events (which came before the other?). To make this comparison possible, the past perfect tense is often used with the simple past tense. Example: "He had left (past perfect tense) by the time I reached (simple past tense) his place." The action of "had left" took place before the action of "reached". There are some people who use time markers in their sentences and omit the use of the past perfect tense. "He left before I reached his place." The word "before" marks the word "left" as an earlier action before "reached". Another reason for using the past perfect tense is to describe a hypothetical (or imaginary) past situation (third conditional sentence). Example: If I had set (past perfect tense) my alarm clock, I would not have woken up late. Lastly, the past perfect tense is used in reported speech. This means we are reporting what someone said when he or she uses the simple past tense. Susan asked, "Have you completed your project?" (Direct speech from Susan) Susan asked if/whether I had completed my project. (Reported speech. Someone reported what Susan asked earlier. Now, we have two past actions "asked" and "had completed". The past perfect tense is required to differentiate their timeline.) As you can see, there are situations which call for the use of the past perfect tense. Back to your sentences: "I didn't notice that he can ride a motorcycle now." (You are referring to a moment in the past when you did not notice his ability to ride a motorcycle. Now you do.) "I hadn't noticed that he can ride a motorcycle now." (Earlier on, you did not notice his ability up to a point when you realised he can. Since you know it now, you cannot use the present perfect tense (haven't noticed) because it would mean that you still do not notice his ability.)