Why is it that Frisian is considered the closest related language to English?

It is commonly asked "What is the closest language to English?" and the equally common answer is

Frisian

Except that there is rarely a reason given for this connection; the most that is given are baldly stated 'facts' (e.g. "English and Dutch are in the same 'Low German' group")

Can anyone give any substantive reasons? For example, an comparative overlap of vocabulary, a distance metric of phonology, privately shared syntactic rules among all the rules in West German languages.

Solution 1:

Kathleen Murphy’s “Frisian, the Language that's Like English” has a good summary of the facts that I've been able to dig up about the relationship between the two languages.

How much alike are English and Frisian? Here's a poem in both languages that shows how similar they can be:

Frisian: Bûter, brea, en griene tsiis is goed Ingelsk en goed Frysk.

English: Butter, bread, and green cheese is good English and good Fries.

The poem is pronounced about the same in either language.... [But] though there are similarities, especially in grammar, English and Frisian speakers generally can't understand each other, which makes them separate languages.

The article shows a “simplified family tree” of Germanic languages with Anglo-Frisian as a direct ancestor of Old English and Old Frisian. While it's “now believed that the hypothesis that Old English and Frisian can be derived from a single Anglo-Frisian mother tongue is an oversimplification” (Hallen, 1998), it's likely that Anglo-Saxon and Old Frisian belonged to a group of mutually intelligible languages. More generally, the Western Germanic languages form a dialect continuum, possibly encouraged by the close trading relationships throughout the long-lived Hanseatic League that made Middle Low German a lingua franca.

Overall, the closeness of the Anglo-Frisian languages is partly from shared vocabulary, and mostly because of how recently they were mutually intelligible. By those standards, linguists actually consider Scots more closely related than Frisian – among those who don't simply consider it a dialect of English.

Solution 2:

I have found a competent article "What is Frisian" by a Frisian native, who is a specialist in languages as well. I gave the link already in a comment, but Mitch asked me to post this as an answer.

The website has some very interesting word chains in Frisian, Dutch, German, and English such as

Frisian     English            Dutch        German

dei            day             dag          Tag

rein           rain            regen        Regen

wei            way             weg          Weg

neil           nail            nagel        Nagel

There are other similar tables on the page, What is Frisian?

Here are some Old East Frisian texts. The navigation from one text to another is a bit cumbersome, the whole website is cumbersome. Use the little square with the yellow arrow. Translations are not given, a pity. Corpus of Old East Frisian Texts

The link does not work like I thought. One can find the Frisian texts, but it's a matter of luck.
It's easier to Google for "old Frisian texts" and choose Titus Texts.

Another interesting link. https://www.fryske-akademy.nl/en/taalweb/

I found the sentence: Hawwe jo fragen? - Have you questions? (German Fragen means questions.)

--- Here's a Frisian poem translated in English. The first line is cited below.

Lyk az Gods sinne weiet uus wrâld oerschijnt;
Like as God's sun sweetly our world o'ershines;

--- Added:
Low German, spoken in the North of Germany beside Standard German, is also very similar to English. Below I give a story about a young woman who had lost her capacity to speak when her mother died in an accident. The story is in Low German and there is a translation in Standard German. I have listed all words of the first section that have a close connection with English. I gave the word in Low German, in English, and in German.

Door there da - Dochter daughter Tochter - dat that dass - jung young jung - hör her sie (personal pronoun, accusative) - deen do tun - wenn when wenn - weren were waren - hör Nam her name ihr Name -weer was war - se she sie - blau blue blau - grot great gross - Ogen eyes Augen - Hoor hair Haar/Haare - wat what was

http://www.plattdeutsche-geschichten.de/geschi31.html

Solution 3:

the vocabulary, grammar, spellings, and syntax are the most similar of all Germanic languages.

This can easily be done by comparing root words, which are very similar in both languages. Words like man, woman, house, child, food, cheese, etc.

I'm not a linguist, but it's no trick to note this. Just compare the grammar and syntax as said, and it's easy to track common roots.

Another thing is that English DNA is very similar to that of the Netherlands and northern Germany. It stands to reason then that the Anglo-Saxons, who came from this region, and founded/made England, shared a similar language to the ancestors of modern Frisians, Dutch, Germans, and Danish. English has many words of OLd Norse origins (leg, freckle, skin, etc.) but then even in Viking times, Old ENglish and Old Norse were very similar.