Hi Chan! Sorry for the delay in my response! I teach on Wed and Fri. (sometimes a 24 hr delay in my responses here)
So Wolhfahrt Op 45 Book 1, No. 1
This is the one that has a down bow on the first note and the next three notes are slurred. Right? If you start at the middle and play regularly, you see you will run out of bow at the frog very soon. So this variation requires you to start at middle bow and play a forceful down bow, getting to the tip, then play the next three notes, saving the bow so that you arrive back at middle or lower middle of the bow. This is a very common bowing pattern in violin literature.
This variation is almost a mirror image of the previous variation,and you will use the same technique but start the bow at the frog and save bow on the slurred three notes, the play a vigorous up bow to regain the starting place on the bow. You can also practice this variation starting in the middle or lower middle of the bow. The teaching point is to use the same amount of bow on the up bow that you use for the slurred three notes, so that you can start each measure at the same place in the bow, without running out of bow. If you practice starting at the frog, save bow on the three note slur, play the 4th note on up bow then ‘follow through’ by jumping back to the frog. You will play the up bow, continue the up bow as the bow leaves the string (like an airplane taking off!) still moving up till you reach the frog. Then set the bow before you play the down bow.
In this variation, only the first note is played alone. All the other notes are slurred in pairs. So what this is teaching you is an ‘overlapped’ bowing. It can comfortably be played middle bow. The challenge is having the slurs a little off the beat, which is sort of like a tongue twister, but I think you will do fine!
Please let me know if you have more questions!