Give a short instruction
Use selected commands and distinguish 'do not' from 'you do not'.
One male addressee
Learn the commands explicitly: اُكْتُبْ write; اُدْرُسْ study; اِفْتَحْ open; اُدْخُلْ enter; اِذْهَبْ go. The initial alif is hamzat al-wasl, not أ or إ. Its starting vowel is u in the three u-stem commands here and i in open/go.
اُكْتُبْ / اِفْتَحْ
Other addressees and politeness
To one female: اُكْتُبِي, اِفْتَحِي. To a male/mixed group: اُكْتُبُوا. مِنْ فَضْلِكَ means please to one male; مِنْ فَضْلِكِ to one female. Requests still need an appropriate tone and context.
اِفْتَحِي / اُكْتُبُوا
A prohibition
Use لا with a jussive present form: لَا تَكْتُبْ do not write (one male); لَا تَكْتُبِي (one female). Compare the indicative statement لَا تَكْتُبُ, you do not write. Here the final vowel or sukun is essential.
لَا تَكْتُبْ / لَا تَكْتُبُ