Please explain and list some examples of words and their primary stresses.

The primary stress in a word falls on the most heavily accented syllable.  There is a primary stress in every word in English. In monosyllabic words, that is, words that have only one syllable, the primary stress falls on the entire word.  In multisyllabic words, it is common practice to define a syllable as a vowel and its surrounding consonants.  Usually this means that any consonants that could be assigned either to the beginning of one syllable or the end of another are assigned to the beginning.  So this means that in a word like slavery, where the stress is on the first syllable, the v is assigned to the beginning of the second syllable rather than to the end of the first.  

If there is a cluster of consonants of three or more, such as in pumpkin, the greatest possible number (according to the phonetic rules of a language) is assigned to the beginning of the next syllable.  In this case, because the combination pk- is impossible in word-initial position, the p is assigned to the first syllable.

Primary stress is commonly denoted in multisyllabic words with an apostrophe (‘) before the indicated syllable.  For the word multisyllabic, for example, the stress would be noted thus:  multisyl’labic.  So we know that the stress falls on the syllable la.

For this exercise I will just put the primary stress in bold, for simplification.

I )  strength
II ) illustrate
III ) distinction 
IV ) slavery
V ) historical
VI ) pumpkin
VII ) shield
VIII ) doubt
IX ) Oxford (x = two phonetic sounds, /ks/, so the three-consonant rule                  outlined above stands)
X ) theatre