На дачу я возьму шорты, кепку и сандалии, лишь бы было удобно гулять.

With дача, Russian normally uses на for going to the dacha and на даче for being at the dacha. It is just the standard idiomatic pattern:

• на дачу = to the dacha
• на даче = at the dacha

Also, на + accusative is used here because there is movement toward a destination.

So:

• Я еду на дачу = I’m going to the dacha
• Я на даче = I’m at the dacha

Because на here shows direction, it requires the accusative case.
The noun дача is feminine, and feminine nouns ending in -а usually change to -у in the accusative singular:

• дача → дачу
• кепка → кепку

So на дачу literally means onto/to the dacha in the accusative form.

Возьму is the 1st person singular future of the perfective verb взять.

• взять / возьму = to take, to bring along, to pick up
• perfective = a completed single action

So я возьму шорты, кепку и сандалии means I’ll take shorts, a cap, and sandals.

A learner should notice that Russian often uses a perfective future where English simply uses will + verb.

Compare:

• Я беру = I am taking / I take
• Я возьму = I will take

Here the speaker is talking about one future decision, so возьму fits well.

They are direct objects of возьму, so they are in the accusative case.

• кепка → кепку
  feminine singular, so the ending changes

• шорты → шорты
  plural inanimate, and for most inanimate plurals the accusative looks like the nominative

• сандалии → сандалии
  also plural inanimate, so accusative = nominative here too

So only кепку visibly changes, while шорты and сандалии happen to look the same as their dictionary forms.

Russian word order is flexible. The sentence starts with На дачу to set the scene or topic first: As for the trip to the dacha...

So:

• На дачу я возьму... puts emphasis on the destination/context
• Я возьму на дачу... is also possible and sounds a bit more neutral

Russian often moves parts of the sentence around to highlight what is most important in context, not just to follow a fixed English-style order.

Лишь бы means something like:

• as long as
• provided that
• so long as
• sometimes if only, depending on context

In this sentence it gives the idea of a main priority:

I’ll take shorts, a cap, and sandals, as long as it’s comfortable for walking.

It suggests that comfort is the only thing that really matters here.

This is a very common thing that confuses learners.

After words like лишь бы or чтобы, Russian often uses a past-tense form in a meaning that is not actually past. Here было does not mean was in the normal past-time sense. It is part of an idiomatic, somewhat subjunctive-like construction:

• лишь бы было удобно = as long as it’s comfortable / provided it’s comfortable

So было here expresses a desired or required condition, not past time.

You can think of it as similar to English structures that are not fully literal in tense either.

Because удобно is being used in an impersonal construction.

Russian often says things like:

• мне удобно = it’s comfortable/convenient for me
• ему трудно = it’s hard for him
• здесь холодно = it’s cold here

So было удобно гулять literally means something like it would be comfortable to walk.

There is no adjective agreeing with я here.
Удобен / удобна / удобно can be an adjective in some contexts, but in this sentence удобно functions as a predicative adverb-like word in an impersonal structure.

The speaker is the understood person, so Russian can leave it unstated.

Fuller version:

• лишь бы мне было удобно гулять = as long as it’s comfortable for me to walk

But since it is obvious from context that the speaker is choosing what to bring for themselves, мне can be omitted naturally.

Russian often omits things that English might make explicit if they are easy to infer.

Yes, гулять is broader than plain English walk.

It often means:

• to walk around
• to stroll
• to go out for a walk
• to spend time outdoors

In this sentence, удобно гулять suggests comfortable for walking around/outside, not just the mechanical act of moving on foot from one point to another.

If the idea were more about the ability to travel on foot in general, another verb might be possible in a different context, but гулять fits well with casual outdoor movement at the dacha.

It can mean either, depending on context. Here comfortable is the best fit.

• удобная обувь = comfortable shoes
• мне удобно в этих сандалиях = these sandals are comfortable for me
• мне удобно встретиться в пять = five is convenient for me

Since the sentence is about clothing and footwear for walking, удобно гулять means comfortable to walk in / comfortable for walking.

Because лишь бы было удобно гулять is a subordinate clause.

Russian normally uses a comma before conjunctions and conjunction-like words that introduce subordinate clauses, including лишь бы.

So the structure is:

• main clause: На дачу я возьму шорты, кепку и сандалии
• subordinate clause: лишь бы было удобно гулять

That comma is standard and expected.

Usually, yes, in normal everyday use they are treated as plural items, much like shorts and sandals in English.

• шорты = shorts
• сандалии = sandals

That is why they appear in plural forms here. Russian does have singular forms in some technical or special contexts for certain clothing words, but for ordinary speech learners should treat these as basically plural nouns.