Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.

This is something I always get confuse with (native language German). As far as I know 'did + verb' is something done and finished in the past. But isn't it the same with 'verb + ed'?.
The normal way of talking about something in the past tense is to use 'verb + ed'. For example, 'I finished the project on time'. You might use 'did + verb' if you wanted to emphasise the point. In the previous example, if someone claimed that you completed the project late, you might say 'I did finish the project on time, and here's the evidence'. Using 'did + verb' where there is no doubt about the truth sounds strange to native English speakers. Saying something like 'I did go shopping for food yesterday' will probably lead to the response 'Well nobody said you didn't!'

Which is correct? The thief carried the knife carefully so as not to cut himself. The thief carried the knife carefully not to cut himself.
The first one. The second one sounds quite awkward, and in fact one way I can think of to improve it would be to add a comma: 'The thief carried the knife carefully, not to cut himself'. The slight pause is meant to suggest the missing 'so as'!

Is there a semantic difference between engine and motor? In some cases, would the use of one or the other word be technically incorrect?.

I like the definitions provided by WordNet and their definition is the way I've understood the difference: A motor is a machine that converts other forms of energy into mechanical energy and so imparts motion. An engine is a motor that converts thermal energy to mechanical work. wordnetweb.princeton.edu So an engine is a specific type of motor. That's why it's not incorrect to speak of a motorboat, or a motorcar, or a motor speedway, even if the boat or car is clearly powered by combustion. Note that if there's no combustion, there's no engine. Purely electric cars don't have engines. As nouns, motor can also refer to a nonspecific agent that causes motion: 'happiness is the aim of all men and the motor of all action', and engine can refer to something used to achieve a purpose: 'an engine of change', a railway locomotive, or a machine used in warfare: 'medieval engines of war'.

