One point of confusion I see in a lot of student scores is a confusion between slurs and ties.
They look very similar. They are both curved lines connecting 2 or more notes. However, they have very different functions. Ties are used to connect two notes of the same pitch, essentially turning them into one note. Slurs are used to connect two notes of different pitches, ensuring a legato performance.
Below is an example of a tie.
This is essentially a single note that lasts four beats.
By contrast, a slur connects two notes of different pitches. Below is an example.
A slur results in a connected and legato performance of two or more different notes. Below is an example of a slur connecting multiple notes.
Multiple notes can be combined with multiple ties. Below is an example.
In the case above, the ties create what is essentially a single note that lasts seven beats.
By custom, each note of a chord is connected in the case of ties. Accidentals are placed before the first note and apply to all subsequently tied notes. Below is a 4-beat chord using a tie.
In the case of slurred chords, only a single slur is used. Below are two chords slurred.
Of course, both ties and slurs can be used in the same passage. In the example below, the last two notes are tied together while the entire phrase uses a slur.
Probably the most common mistake is to attempt to connect notes of different pitches using a tie. This creates an arch that ends in the wrong place.
Two incorrect examples are below.
In the two above cases, a slur should be used instead of a tie.
Of course, using a slur in lieu of a tie can also lead to mistakes. In the example below, a tie would correctly connect each note of the chord. The slur incorrectly connects only the top note.
Similarly, incorrectly using a slur will unnecessarily repeat accidentals.
I hope this helps!