Breakdown of Zawsze mogę polegać na tym przyjacielu, on nigdy mnie nie zawodzi.
on
he / it (m)
ja
I
nie
no / not
ten
this (m)
Questions & Answers about Zawsze mogę polegać na tym przyjacielu, on nigdy mnie nie zawodzi.
The lesson is about polegać na meaning "to consist of" or "to be based on". Why does it mean "rely on" here?
Polegać na is a versatile verb with two major meanings. While your current lesson focuses on it meaning "to consist in" or "to be based on" (e.g., Na czym polega problem? - What does the problem consist of?), its other primary meaning is "to rely on" someone or something, as seen in this sentence. Both meanings grammatically require the preposition na followed by the Locative case.
Why does the phrase use tym przyjacielu instead of the base form ten przyjaciel?
Because the verb phrase polegać na strictly governs the Locative case. For the masculine noun przyjaciel, the Locative ending is -u because its stem ends in a soft consonant (l). The demonstrative pronoun ten must agree with the noun, so it changes to its Locative form, tym.
What is the infinitive of zawodzi, and why is this specific aspect used here?
The infinitive is zawodzić (to let down / to disappoint), which is an imperfective verb. It is used here because the sentence describes a habitual, recurring truth (he never lets me down). If you were talking about a specific, completed failure (e.g., "he let me down yesterday"), you would use its perfective counterpart, zawieść.
Why does the second clause use mnie instead of mi?