Linking words
Words such as "although", "but", "from", "even" and a host of others are usually very important in English, as they are used to show the relationships between the words in your sentences. Getting these small words wrong can alter entirely the sense of the sentence.
Verbs
These can sometimes cause problems in their different forms. There are, for example, quite distinct meanings for the words "can", "may", "must" and "should". If you are not sure, it is best to avoid the construction altogether and say it a different way.