The Terms Kafir and Mushrik Are Interchangeable
1-10-2018 | IslamWeb
Question:
Assalamualaikum.I have multiple questions.1) While I was reading Surah Kahf,I came across ayat 35-36 where a man was arrogant and denying Qiyamah. Some verses later, when his land was destroyed by Allah, he was lamenting saying had he not done "Shirk" this would not have happened. My question is, what he did was disbelief, how is it a shirk?2) Is it allowed to give zakah to a needy person as poltry farm?3) How can I know I am a student of knowledge. By that I mean, in which stage I wont need to to taqlid of a trustworthy Alim, but I can study different Ulamas view and I can choose the most correct one.4) I know several definitions of 'Ijma exist. Which one is espoused by Islamweb?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
As for your first question, the man was a Kafir and a Mushrik [polytheist, someone who associates partners with Allah]; Kufr and Shirk are interrelated. Every Kafir is a Mushrik because he has associated Satan with Allah. It is Satan who has made disbelief in Allah pleasing to him. Every Mushrik is also a Kafir. So, the two terms are intercorrelated even though each term may have its proper meaning.
Shaykh Ibn Baaz 
was asked about the difference between Kufr and Shirk and he replied saying:
“Kufr (disbelief) is to deny the truth and to cover it, like the one who denies the obligation of the prayer, or the obligation of Zakat, or the obligation of fasting Ramadan, or the obligation of Hajj when one is able to perform it, or the obligation of being kind and dutiful to the parents, and so on, such as the one who denies the prohibition of adultery, or the prohibition of consuming intoxicants, or the prohibition of severing ties with the parents and so forth. As for Shirk, it is to dedicate an act of worship to other than Allah, such as the one who seeks refuge in the dead people, or in the absentees, or in the jinn, or in the idols, or in the stars, etc., or offers a sacrifice to them or makes an oath to them. The Kafir is called a Mushrik (polytheist), and a Mushrik is called a Kaafir, as Allah Says (what means):
{And whoever invokes besides Allah another deity for which he has no proof - then his account is only with his Lord. Indeed, the disbelievers will not succeed.} [Quran 23:117]
{Indeed, he who associates others with Allah - Allah has forbidden him Paradise, and his refuge is the Fire. And there are not for the wrongdoers any helpers.} [Quran 5:72]
{That is Allah, your Lord; to Him belongs sovereignty. And those whom you invoke other than Him do not possess [as much as] the membrane of a date seed. If you invoke them, they do not hear your supplication; and if they heard, they would not respond to you. And on the Day of Resurrection they will deny your association. And none can inform you like [one] Acquainted [with all matters].} [Quran 35:13-14]
Allah called their invocation of other than Allah Shirk in the above Surah [i.e. Quran 35], and in Surah al-Mu’monun [i.e. verse 117, Quran 23], He called it Kufr (disbelief). Allah Says (what means): {They want to extinguish the light of Allah with their mouths, but Allah refuses except to perfect His light, although the disbelievers dislike it. It is He who has sent His Messenger with guidance and the religion of truth to manifest it over all religion, although they who associate others with Allah dislike it.} [Quran 9:32-32]
So, Allah labeled those who disbelieved in Him as Kufar (disbelievers) and he also labeled them as Mushriks.
This proves that the Kafir is also called a Mushrik, and the Mushrik is also called a Kafir, and there are many verses and Ahadeeth about this.
For instance, Jabir ibn 'Abdallah 
narrated that the Prophet said: “The distinction between a man and Shirk and Kufr is abandoning the prayer.” [Muslim]
Also, Buraydah ibn al-Haseeb narrated that the Prophet said: “The covenant between us and them is the prayer. Whoever, abandons it has indeed disbelieved.” [Ahmad, Abu Daawud, at-Tirmithi, an-Nasa'i and Ibn Majah with an authentic chain of narrators] [End of quote]
As for the rest of your questions, please ask each question separately so that we can answer it as per the policy of the site.
Allah knows best.