You can collect partial responses to your form using Partial Submit Points. This means that once repsondents pass this point, their answers to the questions before it will be submitted. Add a partial submit point early on in your form, and collect answers to the most important questions, even if your respondents don't fill in the entire thing.
Watch this video to see how to collect partial responses by adding one or more Partial Submit Points to your form.
For more about Partial Submit Points, check out this article: