I am currently reading up on songwriting and am confused by the idea of stress in music. I understand it will be necessary to match the lyrics word stress with the music beat stress but dont know how to identify stress in music almost always. Is there any tutorial or document on the web that clearly explains how to do this by ear?
There are several different kinds of stress in music.
Meter (lyrical) is the stress added by accents (real or implied) in the lyric. It's important to remember that meter, as it was used in writing, actually gave rhythmic form to both poetry and prose. In the case of music, the lyric may not always be delivered in a way which stresses its own meter. In one extreme case, consider the long stretched vowel at the end of the line:
Harmonic stress is the relative or perceived importance of a chord based on its position in a sequence and which beat it falls upon. In a sequence of four chords, assuming the each fall on the first beat of their own measure, the stress pattern is "STRONG WEAK strong weak" where the capitals represent a beat stronger than its lower-case neighbor. This means beats 1 & 3 are the strongest and beats 2&4 are the weakest. This does not refer to loudness of a chord or any technique used to make those beats sound stronger. It just means that they are considered metrically strong based on their position in the sequence.
Beat stress is not discussed alot, but I think this is what you are referring to in matching lyrical rhythm to beat stress. In a measure of common time (with four beats) the stress of the main beats will be the same as above in the harmonic stress pattern. In addition, any subdivided beats (for instance, eighth notes) will be even weaker. For instance, consider:
The effect of this is that you can give undue emphasis to parts of your lyric based on its metric position in the music. However, nobody really sits down and plans on what beat every syllable will land. What you do is come up with an idea, and then try to figure out which syllables sound a little stronger. Yes, they are given strength by either landing on a strong beat or being placed on a long note. But you'll need to develop the ability to recognize this by ear. If you're having trouble you may want to spend some time working on your ear training skills or even studying meter as used in poetry. But actually I suspect you may be overthinking it a little bit and you won't have much trouble. For instance, consider this lyric (with no music to make it simpler):
Based on the words and their order and their meter, which do you think would be given emphasis here? Without music, it's probably why and guy that are being emphasized. But changing the rhythm of the melody can really change this alot. You could stretch out the words "know" and "I'm" (or cause them to fall on stronger beats) which would give them extra emphasis. And that emphasis might change the meaning of your song considerably. At the very least it's safe to say it's awkward to have a junk word which adds no value but has a strong emphasis in the melody. It weakens the song by diluting the importance of the more meaningful words in the lyric.
I think what trips alot of people up here is they're looking for a correct answer. In a math problem we can ask someone and they'll say, yes, that is the correct result. But in writing your own lyrics, you're the authority.