Η αδερφή μου λέει ότι ο βαθμός της ήταν άδικος, αλλά ο δάσκαλος πιστεύει ότι ήταν δίκαιος.

to be

Breakdown of Η αδερφή μου λέει ότι ο βαθμός της ήταν άδικος, αλλά ο δάσκαλος πιστεύει ότι ήταν δίκαιος.

• μου: my
• αλλά: but
• ότι: that

Questions & Answers about Η αδερφή μου λέει ότι ο βαθμός της ήταν άδικος, αλλά ο δάσκαλος πιστεύει ότι ήταν δίκαιος.

Why is λέει (present tense) used together with ήταν (past tense) in the same sentence?

In Greek it’s very natural to mix a present reporting verb with a past situation:

• λέει ότι ο βαθμός της ήταν άδικος
  = she says that her grade was unfair

λέει describes what she is doing now (she is saying/claiming this now),
while ήταν describes the time of the event (the grade was given in the past, the unfairness relates to that past grading).

English does exactly the same:
My sister *says that her grade was unfair.*

If you wanted to place the whole thing firmly in the past, you could also say:

• Η αδερφή μου είπε ότι ο βαθμός της ήταν άδικος.
  My sister said that her grade was unfair.

Could we say είναι άδικος instead of ήταν άδικος? What is the difference?

Yes, you can say είναι άδικος, but the nuance changes slightly.

• ήταν άδικος (was unfair)
  Focuses on the past grading event. The grade was given, and that act of grading was unfair.

• είναι άδικος (is unfair)
  Focuses more on her opinion about the grade now, as a current state.

In practice:

• Ο βαθμός της ήταν άδικος.
  Sounds like: “The way she was graded was unfair.”

• Ο βαθμός της είναι άδικος.
  Sounds more like: “That grade (as it stands now on the paper/report) is unfair.”

Both are grammatically correct; context and speaker preference decide which one fits better.