As salam alaikum I live in the Netherlands and the time of isha is very late and the time of fajr is very early, this causes me to be tired and sometimes with a lack of focus at work. Am I allowed to combine maghrib and isha on work days until the prayer times become more convenient?

Absolutely — here is the revised translation with the proper tone: Al-Imam Ibn ‘Uthaymeen (may Allah have mercy on him) said: If the twilight (shafaq) does not disappear until Fajr begins, or it disappears at a time that does not allow enough time to perform the ‘Isha prayer before Fajr, then such people are like those who do not have a valid time for ‘Isha. In that case, they estimate its time based on the nearest land to them where ‘Isha has a proper time. It has also been said that its time may be estimated according to its time in Makkah, since it is the Mother of Cities. However, if the twilight disappears well before Fajr, leaving enough time to perform ‘Isha, then they must wait until it disappears. But if waiting causes hardship, then it is permissible for them to combine ‘Isha with Maghrib (by praying ‘Isha early with Maghrib), to remove difficulty and hardship. This is based on the saying of Allah: “Allah intends for you ease and does not intend for you hardship” (Surah Al-Baqarah 2:185) and His saying: “He has not placed upon you in the religion any hardship” (Surah Al-Hajj 22:78) In Sahih Muslim, it is reported from ‘Abdullah ibn ‘Abbas (may Allah be pleased with them both) that the Prophet ﷺ combined Dhuhr and ‘Asr, Maghrib and ‘Isha in Madinah without fear or rain. They asked: “Why did he do that?” He said: “He intended not to cause hardship to his الأمة (nation),” meaning that he did not want difficulty to befall them by not combining. May Allah grant everyone success to that which is good and righteous. (End quote from “Majmu‘ Fatawa Ibn ‘Uthaymeen” 12/206)