Final answer:
The two types of stem changes that occur in the present participle are -ing stem changes and -ed stem changes.
Step-by-step explanation:
In the present participle, there are two types of stem changes that can occur. The first type is called an -ing stem change, where a vowel in the stem undergoes a change when the -ing suffix is added. For example, the verb 'run' changes to 'running' in the present participle form. The second type is called an -ed stem change, where certain verbs have an irregular past tense form that is also used as the stem in the present participle. For example, the verb 'go' changes to 'going' in the present participle form.