Questions & Answers about Sygnał jest za słaby, więc nie mogę oddzwonić.
Because sygnał is the subject of the sentence, so it stays in the nominative case.
So the structure is simply:
- Sygnał = the signal
- jest = is
- za słaby = too weak
If sygnał had a different role in the sentence, its form might change, but here it is the thing being described.
Jest is the 3rd person singular form of być (to be) in the present tense.
So:
In Polish, you often omit subject pronouns like on / ona / ono, but you do not usually omit the verb jest in a normal sentence like this.