You may receive guidance that the written work of students with specific learning difficulties, such as dyslexia, should be marked for content and understanding rather than structure and standard written English. Your institution may have guidelines on marking the work of dyslexic students. If not, useful guidance can be found in Accessible Assessments from Sheffield Hallam University, which states
The aim is not to create lower standards or ‘privileges’, but to recognise the strengths and accomplishments of the student.
If you do correct the English, then explain what is wrong with it and why the correction is better.
When marking the work of students with specific learning difficulties you should
write legibly or word-process your comments
consider sending them by e-mail so the students can use software such as Texthelp Read and Write to read aloud the text on the screen
make sure your comments are straightforward and clear
In their marking, some tutors distinguish between content and presentation (spelling, grammar etc) by using different coloured pens.
Take note of the strategies that have been put in place to support the student’s specific learning difficulties, and mark accordingly. For example, if a student has been recently diagnosed as having dyslexia and has not yet had support from a dyslexia tutor, then you may wish to take this into account. Another student may have been receiving support from a dyslexia tutor for some time and you may see the quality of their written work steadily improve as they develop strategies to tackle their difficulties.
In examinations, dyslexic students are likely to receive 25% extra time. Although this can assist with difficulties relating to slower reading and writing and the retrieval of memorised information, it cannot by itself solve deep-rooted language, organisational and memory problems. Be aware that allowing extra time, or making other adjustments, is unlikely to completely level the playing field.