You can say "When you are unable to/When you can't...", the examples you give aren't correct. No worries :)

You should just negate the main verb in the sentence. "When you ARE NOT / aren't able to understand something in English, you can look it up in the dictionary" - nice use of the phrasal verb "look it up" "When you DO NOT understanding something in English . . . " the immediate negation makes it easier for the reader or the listener to understand and it also makes it easier for you to say.

hi there student you can also put it this way 1. ''if you do not understand something in English, you can look it up in the dictionary " 2. "when you do not / don't understand something in English, you can look it up in the dictionary" if you need more assistance/ help feel free to book a class at reasonable price so that we can work on your sentence construction

In day to day conversation “if “you don’t understand something . . . and if you cannot understand something are taken to mean exactly the same, eg. the inability to understand something. However "can't" conveys the definite fact of not having the skill, whereas "don't" implies the ability to understand but leaves open the possibility of being unable to understand something for some other reason. In both cases, you'll definitely want to consult the dictionary to either understand, learn, or verify. Can’t and unable to are both used to talk about ability. Sometimes they are used interchangeably, and sometimes they are not. Can’t is used informally, whereas unable to is used in more formal registers. Consider your situational context and make a decision accordingly. I hope this helps!

"If you can not understand something in English, you can look it up in the dictionary." And "If you do not understand something in English, you can look it up in the dictionary." These are the most natural way of conveying what you are trying to say. There is no difference between the two sentences in the majority of cases. If you would like to use the word "when" in the sentence you can say: "When you are not able to understand something in English, you can look it up in the dictionary."