Phonologists must first determine the inventory, or speech sounds, of a language. The "t" sounds in the words "take" and "steak" are a little different; the "t" in "take" is produced with a little extra breath, or aspiration, while the "t" in "steak" lacks aspiration. These two sounds are called allophones because they are a little different but not enough to change the meaning of a word. Morphemes, such as the "i" sounds in "sit" and "sight," are phonemic; they change the meaning of the word.
Phonologists study the rules that guide the formation of syllables and words. Some languages, like Chinese and Japanese, allow only syllables constructed from a single consonant and a single vowel, or CV syllables. Furthermore, Chinese allows only two-syllable words, while Japanese words can be multi-syllabic. Thus, English words that sound like "strength" are not found in Chinese or Japanese. On the other hand, some languages have consonant clusters that are not found in English. The Czech word "sranda," meaning gag or high jinks, has a consonant cluster ("sr") that does not exist in English.
Phonologists are also interested in the changes that speech sounds undergo based on their position in a particular word. For example, phonologists have found that many languages, such as German and Russian, don't allow voicing at the end of words, so a "d" sound will become a "t" sound. For example the German word for dog is "Hund." It has a "t" sound at the end. In the German word for dog trainer, "Hundertrainer," the "d" is no longer at the end of the word so is pronounced as a "d" sound.
Adding suffixes to words to change their part of speech is called morphology, as when nouns become verbs with the addition of "-tion" ("add" + "tion"). This also changes the pronunciation of the word. The stress shifts to a vowel that is inserted before "tion," and the unstressed vowel reduces to a schwa sound, also seen in tempt/temptation.