Most often a sentence in Korean can be translated word-for-word, down to the most minute details, to its Japanese equivalent, and vice versa. Although each language is considered to be an isolate, there is also a substantial tendency among linguists to place them together in a unique class of their own.
Both use particles which connect words but don't serve much purpose. Some of these particles sound exactly the same and exactly the same purpose. They both rely on particles and verb endings for most grammatical functions, and these systems are similar between the two, though most of the actual words are so different they don't suggest any obvious genetic relationship.
Hanja VS Kanji
Hanja (Traditional characters used in Korea) are recognised by one way. A Korean version of the original Chinese pronunciation from when it was adopted (it may sound more similar to Cantonese) is attached to a hanja. As Korean does not have tones, four words can end up having the same pronunciation.
Kanji (Chinese characters that mix simplified and traditional as well as creating their own unique characters and simplifying some differently than to Mainland Chinese) are recognized by one way, which is significantly more complicated than Korean.
In essence, the Native Korean is like kunyomi and Sino Korean is like onyomi except with differing pronunciations.
The grammar in both languages is almost miraculously similar. Both use extensive honorifics, often regarded as the two most elaborate in the world. Both conjugate their verbs. Both are SOV. Both languages are very similar to each other in terms of grammar. For instance, verbs in Korean and Japanese come after objects, not like English where verbs are before objects (we say "cats hate water" but not "cats water hate").
Logic of expression
For understanding, If korean learn one japanese expression, he or she can make ten other expression easily. With higher success rate. But If Korean learn one English expression, he or she can not make other expression easily, because logic itself is different.
Phonology of both languages is TOTALLY DIFFERENT. Japanese language distinguishes stop consonants by voiced & unvoiced ones. However, Korean distinguishes stop consonants by aspirated - unaspirated ones. Consonants of Japanese language is quite clear, but Korean consonants and even vowels vary in each position.
The number of Japanese vowels was 6 in archaic era, but now is 5. The number of Korean vowels is 8(ㅏ ㅓ ㅐ ㅔ ㅗ ㅜ ㅡ ㅣ). The Kyeonsang dialect of Korean language has 6 vowels system(ㅏ ㅓ ㅔ ㅗ ㅜ ㅣ), quite similar to the archaic vowel system of Japanese language.
However, deeper studies show that two languages can't be the same according to the cultural and environmental matters. The combinations of sounds and their pronunciation are quite different. It is easier for a Korean to learn to pronounce Japanese, than it is for a Japanese to learn to speak Korean.
This article is based on my personal experience and my mini research of being a language practitioner in both languages. If you are a beginner trying to achieve these two languages, I am sure that this will give you clearer ideas.