Hi @joanofdart,

Glad to see you in Github Community Forum!

To answer your queries:

I want to stop a job from happening if the previous job didnt happen.

Jobs run in parallel by default. To run jobs sequentially, you can define dependencies on other jobs using the jobs.<job_id>.needs keyword. For example:

jobs:
  job1:
  job2:
    needs: job1

In this example, job1 must complete successfully before job2 begins, if job1 fails, the job2 will not begin.

• How do I expose a variable from the previous job?
• How do I check for the value of this variable?

Jobs are executed on individual VM, you can transfer the variable value to the next job which needs the previous one.
‘set-ouput’ in previous job and get via jobs.<jobs_id>.outputs. in the 2nd job, please check the sample here for more details.

• Does a single if: blocks everything after it or just the next line?

The if expression can be defined in step or job level, it only affects the step or job where it defines, not affect the next step or jobs.
Please check the link for more details.
Job level:

  <a href="https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions" target="_blank" rel="noopener nofollow ugc">GitHub Docs</a>

Workflow syntax for GitHub Actions - GitHub Docs

step level:

  <a href="https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions" target="_blank" rel="noopener nofollow ugc">GitHub Docs</a>

Workflow syntax for GitHub Actions - GitHub Docs

Hope it clear and helpful!