In plain words
We must make a fitting effort to keep our surroundings clean as well. Then inner purity. The mind grows foul through attachment and hatred. When evil feelings enter, the mind grows fouler still. In a stained mind no pure feelings at all will arise. So, to make the mind pure, we must cultivate sacred feelings such as Prema, compassion, forbearance and sympathy.
Then the second, tapas. It is not merely putting the head down and the legs up and sitting in some posture or other. It is to dwell on the goal you desire, constantly, with the purity of the three instruments, so as to attain it. That very tāpa turns into tapas. “I must achieve such and such a goal”: so you must keep remembering it constantly, at all times and in all states. That is why they said, “Satataṃ yoginaḥ.” To attain the principle of the Ātma, whose nature is without attributes and without form, we must burn with yearning. That very paritāpa turns into tapas. What you think in the mind and say in word, you must carry out in action, and experience their oneness in action. This is the purity of the three instruments. That is the contemplation of God. By such tapas there is a chance to attain the supreme principle, without attributes and without form. To leave the village, go to the forest, graze on leaves and roots and waste one's life is not tapas. To drive away the bad feelings within us: that is tapas.
To leave property and wife and go to the forest is not it; to give up the bad feelings within one’s mind: that alone is yoga, and that itself is tyāga.
To purify the heart through such sacred feelings is tapas.
Then santoṣa. When does santoṣa come? Only when there is contentment does man find santoṣa. The more our greed grows, the more discontent alone grows in us. It brings sorrow. That is why we must be content with what we have. We must enjoy Ānanda with what we have got. We must not desire desires that go beyond the limit. Only when we are content do we find Ānanda. He who has much satisfaction is the richest man, he is a satisfied man. That is why we must stay content. Through that very contentment we find Ānanda. Then the fourth, svādhyāya. Svādhyāya does not mean only studying the Veda.