TABLE OF CONTENTS
You have previously learned that "Nounを + transitive verb" describes an action taken toward something else whereas "Nounが + intransitive verb" expresses the subject taking an action without directly affecting something else. When they are used in the ています form, a transitive verb + ～ています expresses an action in progress and an intransitive verb + ～ています expresses a state resulting from a change, respectively.
ex. ドアを開けています。(I am opening the door.)
ドアが開いています。(The door is open (as a result of a change).
Some of the intransitive verbs in ～ている form are not as straightforward as other verbs such as ドアが開いています, 電気がきえています, etc. Because they indicate states of being, you might just want to use ～があります instead of the appropriate intransitive verbs. However, using the appropriate intransitive verb + ている makes your Japanese sound a lot more natural like a native speaker's. The common examples of such verbs are the verbs 出ています and おちています.
ex. 犬（いぬ）が外に出ています。(The dog is outside.) → If you translate this English sentence back into Japanese, you would probably say 犬が外にいます, but if you have let your dog go outside temporarily, you should be using the former since 犬が外にいます only describes that a/the dog is outside.)
お金がおちています。(There is money (left) on the ground.) → お金 is not supposed to just be on the ground. If you see something left on the floor/ground, you always need to use おちています instead of あります.)
This is not directly related to the target grammar, but remember the use of は-が construction for "whole-part" relations.
ex. Aの部屋はまどが開いていますが、Bの部屋はまどがしまっています。(Instead of Aの部屋のまどは開いていますが、Bの部屋のまどは…）
The grammar "てform + しまう" has two usages, which are (1) expressing something that happened being unfortunate/regrettable or (2) describing something one did early, quickly, beforehand, and/or with determination. The latter use of ～てしまう is often accompanied with words such as もう, 早く, ～までに, etc.
ex. (1) 服をよごしてしまいました。(I regrettably/accidentally made my clothes dirty.)
(2) 明日は時間がないから、今日中に宿題をやってしまおうと思います。(Since I don't have time tomorrow, I will finish the homework within today.)
The casual equivalents of ～てしまいました are ～ちゃった & ～じゃった, and they take the "Pre て form" + ちゃった・じゃった (i.e., if the てform ends with ～て, the verb takes ～ちゃった, and if the verb ends with ～で, it takes ～じゃった). Remember that we use not only the past tense of these for expressing the regret but also the non-past tense for things we intend to finish early/completely.
ex. 転んでけがしちゃった。(I (regrettably) injured myself by tripping.)
今日中にこの部屋かたづけちゃおう。(Let's tidy (finish tidying) up this room by the end of today.)
In semi-formal speeches, we also use ～ちゃいます（ました） & ～じゃいます（ました）.
ex. この仕事、午前中にやっちゃいましょう。(Let's finish this work in the morning.)
部長、この書るい、すてちゃってもいいですか。(Department Manager, is it OK to throw away this document?)
The sentence pattern "Aと、B" with Conditional Form と indicates that when(ever) Condition A meets, B happens, which is similar to たら sentences. However, unlike たら, と is used only for non-volitional sentences and cannot express plans/will/desires, etc. It cannot be used for invitations and suggestions, either.
と conditional form expresses "Cause-Effect" sentences.
Used only for non-volitional sentences, and often used for natural phenomena/sequence, manuals, and giving directions.
Can also express your hope for good results （～といいですね） and assuring a certain result together with 大じょうぶですよ, あぶないですよ, 元気になりますよ, etc.
ex.
〇 ホラー映画を見ると、（いつも）夜ねられません。 (Whenever I watch horror movies, I cannot sleep at night. = expected consequence)
〇 このボタンをおすと、おゆが出ます。(When/if you press this button, hot water comes out. = manuals)
〇 そこに入ると、あぶないですよ。 (It will be dangerous if you enter there. = assuring a certain result)
〇 はるになると、はながさきます。 (When(ever) spring comes, flowers bloom. = natural phenomena)
= はるになったら、はながさきます。
✖ はるになると、ピクニックに行きましょう。 (UNGRAMMATICAL; と cannot be used with volitional sentences.)
〇 はるになったら、ピクニックに行きましょう。(When spring comes, let's go on a picnic.)
Pay attention to the differences among と・たら・時.
たら can be used for almost all situations where と is used. However, たら is more colloquial than と, so it's not used for manuals (i.e. in sentences such as 'if you press this button, the machine will turn on' in a manual booklet/written sign, etc.).
Both と and たら are "A then B" sentences. 時 can be "A then B," "B then A," or "A and B at the same time," depending on the verbs and tenses used in the A part.
In the past tense, both たら and と have limited usages, which is that they can be used (1) for habitual actions/recurring events or (2) for single unexpected events.
ex.1 小さい時、ふゆになると／なったら、よくみかんを食べました。(When I was small, I often ate tangerine oranges in winter (=when it became winter).)
ex.2 ピクニックに行くと／行ったら、大雨がふって来ました。(When I went on a picnic, it (suddenly/unexpectedly) started raining heavily.)
When you use any of the と・たら・時 sentences, think about whether or not you can use the other two for the same sentences.
"AながらB" describes two simultaneous actions taken by one person, and takes the Pre-ます form of verbs.
ex. よく歩きながら考えます。(I often think while walking.)
電話を使いながら運転するのはあぶないです。(It is dangerous to drive while using your phone.)
Remember that the B part (=main clause) usually has the main action although the equivalent English sentence can be different.
ex. 音楽を聞きながら宿題をします。(I listen to music while doing homework.) → Doing homework is the main action.
テレビを見ながらご飯を食べてはいけません。(You must not watch TV while eating.) → Eating is the main action.
ながら can be used not just for short term actions but also for long-term activities as in the following examples:
ex. 働きながら、勉強しています。(I'm studying while working (full-time).)
旅行しながら、写真をとります。(I (will) take pictures while traveling.)
ばよかった, literally meaning "it would have been good if I had/hadn't done...," uses Conditional Form ば to express the alternate course of action you did not take. As for the conjugations, use the dictionary form as the base and change -u to -eば for うverbs, and change る to れば for る verbs. For irregular verbs, する becomes すれば and くる becomes くれば, respectively.
ば-form Conjugations
ex. ほしかった服をきのう買えばよかったです。(I should have bought the clothes I wanted yesterday.)(You can also say きのう買っておけばよかったです。）
きのうのばんおそくねなければよかったです。(I shouldn't have gone to bed late last night.)
Remember that you usually use an action that you have control over with this; otherwise, it sounds a bit unnatural.
If you have overslept and regret about it, say:
早くねればよかったです。(I should have gone to bed early.)
NOT 朝ねぼうしなければよかったです。(I shouldn't have overslept. - 朝ねぼう is something you accidentally do, so you cannot use it with this pattern)