Question
I have been learning English for a few years. But I am so sorry that I don't know how to pronounce some simple words like "months," "sixth" and "Smiths." Would you please tell me how to pronounce them as clearly as possible?
Answer
Firstly, note that all the words you have cited contain the /th/ sound.
This is a sound which — I am sure you are aware — doesn't exist in Chinese, and doesn't exist in many other languages either.
My previous experience with Chinese learners in teaching this sound is that the difficulty in pronouncing /th/ does not come from difficulties in placing the tongue between the teeth, or just behind the top teeth. Most Chinese students are aware of putting their tongues in the right place.
The difficulty seems to come from an inability to spread the mouth wide enough to make this sound.
Try to relax your jaw and spread your lips when saying /th/ (it is much easier to show you than to write to you about it!), otherwise you will not pronounce it well.
Secondly, the words you have cited contain what we call "consonant clusters." What this means is that consonant sounds are put close together (eg: k — s — th in "sixth").