Learn Turkish: Herkese safça güvenmemek lazım. - It is necessary not to trust everyone naively.

The word saf means pure or naive. The current lesson's suffix -ce / -ça can turn adjectives into adverbs. Here, it attaches to saf to create safça (naively), showing how the trusting happens. Notice that because saf ends in the voiceless consonant f, the suffix mutates to start with ç instead of c, and it takes a due to vowel harmony.

Why is there an -e at the end of herkese?

The verb güvenmek (to trust) requires its object to take the dative case (the to/towards direction). In Turkish, you literally trust TO someone. Therefore, herkes (everyone) takes the dative suffix -e to become herkese.

Why is the verb güvenmemek in its infinitive form instead of being conjugated for a specific person?

The word lazım means necessary. When you want to state a general rule without pointing at a specific person (like I, you, or they), you simply use the dictionary/infinitive form of the verb followed by lazım. To make it negative, the negative suffix -me goes right before the infinitive -mek, creating güvenmemek lazım (it is necessary not to trust).