In theory, the tonic syllable is the only one that carries a movement in pitch; therefore it can be identified relatively easy. That is in theory! In practice is not that simple.
As it is well known, if there is a tonic syllable followed by a tail then the tone is carried by the tonic syllable and the tail. In many such cases the pitch movement is extremely difficult to be detected on the tonic syllable itself.
It is also claimed that one of the tones is the level tone, which does not have any pitch movement. In this case, the tonic syllable is identified as the most prominent syllable.
Sometimes, a tone unit contains two tonic syllables; usually first syllable has a fall on it and the other a rise as is shown below.
At the first look the two examples look similar but they are actually different. In the first example the word “him” has a greater importance than in the second example, and it can not occur in its weak form /ɪm/, but must be pronounced /hɪm/. In the second example the pronunciation of “him” is very likely to be /ɪm/ so “I’ve seen him” is phonemically translated as /si:n ɪm/.
The two versions also have different meanings. Let’s take a look at the following conversation:
A: John is a really good player.
B: Oh yes, I’ve \seen /him.
As you can see, version one can be used in conversation on hearing someone’s name. On the other hand, version two gives the word “seen” a much greater importance than in version one and it sounds like the speaker has something further to say. Let’s take a look at the example below:
A: Have you seen my brother yet?
B: I’ve \/seen him, but I haven’t had time to \talk to him.
All these, weaken the general rule that says that each tone-unit contains only one tonic syllable.