My changes, as usual, appear in brackets. Let me know if anything is unclear.
Spanish and Japanese [also] differ from each other in their number of tense forms, but both of them use conjugation to indicate tenses. There are three tense forms in Spanish: the past tense, the present tense and the future tense. For example, to form the present simple tense, it uses conjugation [to designate] auxiliary verbs, as in hablo means ‘I speak’. On the other hand, in Japanese there are only two basic tense forms: [the past has its own form, while both the present and the future have the same tense form]. For instance, the gerund (-te form) plus the auxiliary form imasu/iru are usually use[d] to indicate the present [and] future tense: as inkite imasu regularly means ‘I have come’. English is different from [both] Spanish and Japanese in that it [uses] inflections for tense and word formation. The past tense, for example, is expressed by ‘ed’. Furthermore, prefixes and suffixes can be added to form new parts of speech such as ‘available’ (adjective) and ‘availability’ (adverb).
In Spanish[, the] subject and verb agree in person and number. This means that [the] subjects and the verbs they go with are arranged in conjugations [that take] six forms: 1st, 2nd, and 3rd person, each used in singular and plural. Moreover, noun[s], articles and adjectives have to agree in gender (masculine and feminine), and number (singular and plural). [However,] English has only inflections for number agreement. For instance, subjects and verbs must agree in number as in ‘I go…, He goes’. [Finally], Japanese is unlike Spanish and English in that derived form[s] of word[s] occur often in Japanese. [For example,] [n]ouns can be made into verbs, adjectives into nouns, gerunds and other forms, and so on. There is no subject-verb agreement rule b[e]cause the same form for the verb is used with singular and plural subjects. There is also no [number] distinction[;] for instance, the noun ki can be ‘tree’ or ‘trees’. Where number is important, it can be indicated by providing a quantity (often with a counter word) or (rarely) by adding a suffix. Moreover, [the] plural, gender, and articles [do not occur] in Japanese.
[The grammars of all three languages also include] the concept of grammatical mood, but they differ in amount of the moods. Moods indicate the feeling of the verb; more specifically, the speaker's attitude or feeling toward the action, e.g., true, uncertain, possibility or command. In Spanish[,] [there are] four moods: indicative, conditional, subjunctive and imperative, while in English [there are] only three moods: indicative, subjunctive, and imperative. Japanese has more moods than Spanish and English, [including the] imperfective, continuative, terminal, attributive, hypothetical and imperative. For example, the imperative mood is used to expresses direct commands, requests, and prohibitions: "Paul, do your homework now" is a case in point.
In Spanish, subject pronouns are very often omitted due to the [fact that the] verb forms themselves usually signal what the subject is. [Unlike in English,] It is not necessary for comprehension [to include them], because the subject of the verb is evident from the conjugated verb. [For example, the verb] ‘I go’ can be translated by ‘yo voy’ or simply ‘voy’. Another difference is regarding [the] position object pronouns take within the sentence. The object pronouns in Spanish always go before the verb, whereas in English they always go after the verb[.] For example, Le dije and I told him [are correct constructions in Spanish and English, respectively]. Japanese contrasts with Spa[nish] and English [in that] pronouns [are] usually used when the speaker wants to [emphasize a particular point, since pronouns are] scarcely used in Japanese, [and in everyday use, it is better to repeat the names than to use pronouns.]
Some of these differences may explain why people have trouble learning any of these languages. Differences in pronunciation and grammar are usually the most difficult to overcome, but [fortunately are] not impossible.