Real Obstacles: nawet jeśli
When training for a tough race, you have to push through real physical challenges. To say "even if" for situations that are real, likely, or factual, use nawet jeśli.
Because these sentences deal with reality, the verbs in both parts of the sentence remain in the indicative mood (your standard present, past, or future tenses).
Hypothetical Obstacles: nawet gdyby
When talking about imaginary, impossible, or highly unlikely scenarios, use nawet gdyby (even if).
Because you are dealing with a hypothetical situation, the verbs in both the condition and the main clause must take the conditional mood.
Conditional Endings and Sentence Order
Remember that just like the base word gdyby (if), nawet gdyby absorbs the personal conditional endings for "I", "you", and "we". For example, "even if we had" becomes nawet gdybyśmy mieli.
Finally, you can freely flip the order of the clauses. The nawet jeśli or nawet gdyby clause can easily be placed at the end of the sentence to emphasize your determination.