I'm trying to figure out which hand(especially which finger) I should use for notes in some measure in sheet music that I'm currently practicing. I attached a screenshot below. As you can see, There is G7 chord, I can use left hand like G-D-B-D, or is it okay just use left hand for G-D(finger 5, 1) and right hand for BG and finally left hand D or I must use right hand for all these notes(in treble clef) except G in bass clef? I know this is silly question, I'm a beginner who learning piano myself and I love music so much. so any comments and answer would be really appreciate!
As a generalisation, r.h. - treble clef notes, l.h. - bass clef notes. Often, as here, the composer or publisher will try to be helpful, with clues like the stem direction showing which hand should play which notes.
However, if you find a different, easier way, that doesn't give wrong emphasis to certain notes, why not use it? Heck, even in exam situations, the examiner does not give any weight to what fingerings are being used.
Personally, I'd probably play Bs and Gs all with r.h. - the same two repeat - and the Ds with l.h.
The upper staff is generally for the right hand and the lower for the left, but this is not a strict rule. There are various mechanisms for indicating exceptions. In this case it is fairly clear that the downward-pointing stems are for the left hand. This follows from several facts: it is a musically coherent phrase beginning in the bottom staff; when this phrase moves to the top staff, the bottom staff is empty (including a lack of rests); and there is also a higher phrase keeping the right hand busy.
The fact that there are two notes assigned simultaneously to both hands, however, suggests that this is a piano arrangement of a piece for band or orchestra. These arrangements often contain extra information such as this. It serves to tell you what the band is doing in more detail; this would be lost if the arrangement were written more strictly as it should be played on the piano. (In other words, it isn't likely a good idea to play those notes with two fingers, even though the fingering calls for it.)
Sometimes, playing all the treble-clef notes with the right hand only without letting go of notes earlier than notated or needing pedal is impossible, especially with your hand size. In that case, play some of those notes with your left hand - you personally have no choice.
That red section of notes is the case for me - my hands each span only an octave comfortably and a 9th with mistakes or inordinate amounts of time taken to adjust the hands, so there's no way I can play both the half-note high G and the lower D above Middle C at the same time with my right hand only. I must use the left hand for that D.
Also take the fingering on the sheet music into account in that red section: it asks for a ring-into-pinky finger for the high G and a ring finger for the D above Middle C at the same time. Pulling that off with the pinky and ring fingers of the right hand only is pretty much impossible without letting go of notes early.
Admittedly, the red-outlined passage does have pedal, so you are theoretically allowed to let go of notes early there and other such shortcuts. But make it as easy as possible to play, yet needing as little pedal as possible to pull off the notated music.