Because stress is so important, there are several signals to make the stress pattern clear to the listener, but lengthening is perhaps the most important, and probably easiest to teach. Following are crude stress rules for beginners . The vowel in a strong syllable is long. Some vowels keep their regular sound, but some vowels get weak. A weak vowel sound is short and not clear.
Most weak vowels sound the same.
Actually, there are three different weak vowels, but from a beginning student's point of view, their obscurity makes them sound pretty much alike. Probably the most essential element of this system is the lengthening of the vowel at the center of the primary stress of the focus word. The purpose of the lengthening is to make it easier for the listener to notice the pitch change which occurs when that particular word is highlighted in the sentence. Contrastive stress is probably the most common form of this use of emphasis. Following are my rules for beginners.