What do we mean when we say that word stress in English is unpredictable?

With English language, and perhaps few other ones, not all the syllable (letters) are said with the same strengthen of voice. Sometimes a word can be stressed loudly while others can be reduced and said quietly. However, it is really hard to predict where the stress will be on a word and here are two examples:

Let’s take the three words which are very similar: photograph, photographer and photographic. You would expect the stress on the same letters of different words to be the same and follow the same patter but that is not the case.

• Photograph: the stress is on the first part of the word (pho)
• Photographer: the stress is on the second part of the word (to)
• Photographic: the stress is on the end of the word (graph)

So it is really hard to predict where the stress will be by just looking at the letters.

Another example are the similar words: ability and able.

The word ability has more stress on the middle and last part of the word (bility) while the very similar word able has more stress on the first part (a). This is very unpredictable.