Praise be to Allah.
Importance of offering prayer at their fixed times
The basic principle is that each prayer should be offered at the time appointed for it by Shari’ah. Allah says (interpretation of the meaning):
“Verily, As-Salah (the prayer) is enjoined on the believers at fixed hours” [al-Nisa 4:103]
Can you pray ‘Isha early?
Joining two prayers is not permissible unless there is a reason such as travelling, rain or hardship. If the time for ‘Isha is very late and the time for Fajr is early, such that it will cause hardship to offer ‘Isha on time, in that case there is nothing wrong with joining it to Maghrib at the time of the earlier prayer.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) was asked about a country in which the disappearance of the red afterglow which signals the onset of the time for ‘Isha comes very late and it is too hard for them to wait for it.
“If the red afterglow does not disappear until dawn comes, or it disappears at a time when there is not enough time to pray ‘Isha before dawn comes, then they come under the rulings of those who have no time for ‘Isha. They should estimate its time based on the nearest country to them where there is considerable time to pray ‘Isha , and it was also suggested that they should base it on the prayer times in Makkah, because it is the mother of cities (Umm al-Qura).
If the red afterglow disappears a long time before ‘Isha and there is enough time to pray ‘Isha, then they must wait until it disappears, unless waiting for it will cause them hardship. In that case it is permissible for them to join ‘Isha with Maghrib at the time of the earlier prayer, so as to ward off hardship and difficulty, because Allah says (interpretation of the meaning):
“Allah intends for you ease, and He does not want to make things difficult for you.” [al-Baqarah 2:185]
“and [Allah] has not laid upon you in religion any hardship.” [al-Hajj 22:78]
In Sahih Muslim, ‘Abd-Allah ibn ‘Abbas (may Allah be pleased with him) narrated that the Prophet (peace and blessings of Allah be upon him) joined Zuhr and ‘Asr, and Maghrib and ‘Isha in Madinah at a time when there was no fear and no rain. They said: What did he intend by that? He said: “He intended that his ummah should not be put in difficulty” i.e., that they should not be faced with hardship due to not joining prayers. May Allah help us all to do that which is good and right. (Majmu’ Fatawa al-Shaykh Ibn ‘Uthaymin, 12/206)
And Allah knows best.