Phrase structure and harmonic progression require a great deal of study to understand fully. However, you can use careful listening and your musical intuition to identify the beginnings and endings of phrases.
As you listen to the hymn to the right, note how much stronger the cadences feel after measures 4 and 8 (large breath marks) than those after measures 2 and 6 (small breath marks). The two stronger cadences divide the hymn tune into two 4-measure segments or phrases. The 4-measure phrase is a rather standard feature in much of the music we commonly hear, including hymns.
The cadences after measures 2 and 6 can be considered intermediate (“half”) phrases, and will probably not need to feel as final as those after measures 4 and 8.