Flattery vs Being Nice

Question

Flattery is forbidden

talmud:

Rabbi Elazar says: Any person who has flattery in him brings wrath to the world, as it is stated: “But those with flattery in their hearts bring about wrath” (Job 36:13). And moreover, his prayer is not heard, as it is stated in that same verse: “They do not cry for help when He binds them.”

There are exceptions for example your wife for peace, you teacher to teach you and for peace (to protect yourself)
Pele Yoetz

flattery our Sages allowed towards a person’s wife, for the sake of domestic peace, and towards his rabbi, so that he should teach him Torah. A person is also allowed to flatter wicked people due to the ways of peace.

but it is good to be nice (to say what people what to hear, even when it is not "true") always and to all peaple/creations
talmud:

Beit Shammai said to Beit Hillel: In a case where the bride was lame or blind, does one say with regard to her: A fair and attractive bride? But the Torah states: “Keep you from a false matter” (Exodus 23:7). Beit Hillel said to Beit Shammai: According to your statement, with regard to one who acquired an inferior acquisition from the market, should another praise it and enhance its value in his eyes or condemn it and diminish its value in his eyes? You must say that he should praise it and enhance its value in his eyes and refrain from causing him anguish. From here the Sages said: A person’s disposition should always be empathetic with mankind, and treat everyone courteously. In this case too, once the groom has married his bride, one praises her as being fair and attractive.

rashi

לעשות לאיש ואיש כרצונו

rambam

הָיָה דִּבּוּרוֹ בְּנַחַת עִם הַבְּרִיּוֹת וְדַעְתּוֹ מְעֹרֶבֶת עִמָּהֶם
his social conduct is [attractive] to others

what is the difference? (when is it being nice when is it flattery?)
sources please

Answer 1

Here is a quote from an article about the prohibition of flattery:

Although the word chanufah in Modern Hebrew means "flattery," and, indeed, is even occasionally used by Chazal in this sense, the prohibition against chanufah has a slightly different meaning. Chanufah is the deception that occurs when someone encourages the performance of misdeeds, aveiros, or when someone fraudulently misrepresents something as Torah or as acceptable behavior when it is not. The primary case of chanufah is when someone sees or knows that a person sinned and tells the sinner that he did nothing wrong or, worse still, tells the sinner that the sinful act was the correct thing to do.

In other words, the forbidden flattery you are inquiring about is limited to cases which can cause misunderstanding of the Torah or justify bad behavior. Otherwise, there's no prohibition in being nice to people.

The article goes on to delineate different forms of flattery and other aspects and applications of the prohibition; I think you will find the sources mentioned there helpful.