Stressful circumstances may force a young person to grow up quickly.
A foil in literature describes a character who contrasts with another character in order to make comparisons and highlight certain traits between these two persons.
Both Melantho and Eurycleia are servants of Odysseus. While Melantho is unloyal, (sleeping with the suitors while Odysseus is away) and treating him rudely when he returns disguised as a beggar, Eurycleia remains devoted and kind. Melantho is used as a foil for Eurycleia to showcase her good traits.
In Things Fall Apart, the arrival of the missionaries best illustrate the struggle between tradition and change.
While many resist the changes the missionaries bring, many like them. The missionaries bring new goods to the village and export village goods, bringing money into the village. This trade, however, has its drawbacks. Meanwhile, conversation between village elders and Mr. Brown allows them to understand one another. Mr. Brown tries to encourage the elders to educate their children, explaining this will bring hope for their futures.
Although many welcome these changes, others -- including Okonkwo -- are resistant. He and a few others do not like this change and even openly resist it. They value their traditions and do not want them altered.
Therefore, of the many themes in the novel, the one represented by the arrival of the missionaries is "the struggle between tradition and change."