A change in basic sentence order can make an English sentence nonsensical or make it mean something completely different. For example, if part of the verb is moved so it comes before the subject, we have a question. (e.g. "John will be home at eight" becomes "Will John be home at eight?") If you switch the time phrase with the place word you get an understandable sentence that no native speaker would ever say, "John will be at eight home." Word order is also important in English with phrases smaller that a sentence. For example, notice how natural phrase #1 sounds and how unnatural phrase #2 sounds:
Present tense: The present tense in most language refers to actions that are taking place in the present. In English, this is not really true. We use present tense to refer to actions that are habitual, repeated, or always true. (e.g., The sun rises in the East; I get up every day at 6:00 a.m.; We celebrate Thanksgiving in November). English uses present progressive (present continuous) to express actions that are taking place in the present (e.g., I'm reading a teacher training manual; I'm teaching an ESL class; You're preparing to take your GED).
Future tense: The most common future tense in English does not use WILL as you may have been taught. The most common future tense is produced with the expression GOING TO (e.g., I'm going to eat . . .; he's going to show us how . . .; we're going to study . . .). You should remember also that in everyday speech this GOING TO expression is pronounced "gonna" and that it is not incorrect to say it that way as long as it is understood.
Two-Word Verbs: Some actions in English are expressed by phrases that consist of a verb and a preposition or adverb. The action is not expressed by the verb alone. For example, GET means 'to obtain, to acquire, to receive, etc.' while GET OVER means 'to recover from an illness,' and GET UP means 'to arise from a reclining position.' There are hundreds of such phrases in English. They are a problem for ESL students because they are often not listed in the dictionary in a separate form and their meaning is hard to find. A good textbook will probably teach many of these two-word verbs, but if yours does not, you should teach them to your students as they arise naturally in the classroom (for example, HAND IN your papers).
Besides not appearing as separate entries in the dictionary, these two-word verbs present one other problem. Some of them must have their two parts together in a sentence while others may have their parts separated by other things in the sentence.
NOUNS -- In English, as in many other languages, we consider some things countable and some things non-countable. If something is countable, it can have a plural form. If it is non-countable, it can not have a plural form and the singular form is used to refer to any quantity. Some of the things that we consider non-countable in English are: abstractions, ideas, ideals, emotions, gasses, fluids, materials with particles too small to be conveniently counted, and fields of study. Many languages have the same concept of count and non-count but they do not always put items in the same category as we do in English. For example, in English, HOMEWORK and HOUSEWORK are generally non-countable. (We do not say HOUSEWORKS, for example.) In other languages, these nouns are countable and they do have plural forms. Students have to learn which nouns are which in English because it affects other grammar principles as well (e.g. whether to use A LITTLE or A FEW before the noun). There are other problems with English grammar but most good textbooks can guide you along. The pointers included here are mentioned only to make you aware that there are many aspects of English that are not problems at all for native speakers (even uneducated ones), but which might cause problems for your students.